Why doesn't Bridgestone, Michelin, Hankook, or Continental make ST type tires?

I was asked by a fellow RV owner if I could recommend a tire for their trailer.

We have asked why the major tire companies other than Goodyear (Bridgestone, Michelin, Hankook, and Continental) don't carry ST tires, but they didn’t really provide a direct answer.  Just sort of danced around it.  I have been reaching out and trying to find an ST provider, but have not had any luck so far.  

My reply:  In My Opinion,

None of the "major" tire companies are interested in the RV trailer market (ST type tires) because of the documented poor level of weight and inflation management. Goodyear tire company pushed for the introduction of Special Trailer tires back in the late 60's & early 70's at a time when 20' single axle trailers pulled with bumper hitches were the norm. a 1/2 ton truck was rarely an option for average family use vehicles. ST tires were offered with increased load capacity i.e. lower cost to the RV company and to compensate for this the 65 mph max speed and reduced tread depth was presented as required and sufficient to allow the bias tires of the day to run to wear out in maybe 10 to 15,000 miles. Many times P-type passenger tires were applied to these smaller RV Trailers but a -10% load capacity adjustment to the P-Type load capacity was required.

The 1974 National 55 mph speed limit kept operating speeds down so tire failures were still at a lower level but with the introduction of steel-belted radials to the tire marked coupled with the growth of trailers to 35'+ and tire mileage theoretically increased to the 30 or 40,000-mile range the tires no longer wore out but started to fail at an unacceptably high rate.

The annual tire market in the US is 200 Million a year for Passenger type tires and about 37 Million LT type. Annual sales of all types RV is very volatile and ranges from 160,000 to 500,000 a year for all types, from Class-A down to teardrop,  Some industry information suggests only 20% of the RV market are "conventional RV Trailers so that would translate to a market of 400,000 tires in the best years  When looking at the tire market I think we can see little incentive for a company to go to the effort and expense of developing tires for a market that is only 0.5% of the P- and LT-type tire market. Especially considering the high level of warranty claims and problems due to the extremely poor maintenance record seen from RV owners.

BUT   back to the original question. The only ST tire that I was able to look at a section of recently was a Goodyear Endurance. It looked like it might perform better than some other ST tires I have looked at.  BUT  the only performance data I have is just the reports on various RV Forums where customers seem happy with the Endurance.

Sorry to say that your choice is relatively limited if you have to stay with St type tires. While Maxxis and Cooper have supporters you might be better off switching to LT type as long as you make the required adjustments for load capacity by increasing physical size and or Load Range. If you follow my blog you should have already see THIS post of the topic of lT vs St type tires..

Heat, High Speed and the "Magic" in ST tires.

Heat generation is primarily the result of a combination of High Speed, High Load, and Low Inflation. Factors that can counter some of the negative effects of heat on tires can include the use of more expensive rubber compounds, reduced thickness tread and other construction features that are out of our control as tire owners. Without access to secret formulas and other proprietary information, I know of no way to learn which of these features is in which tires if any. There is one exception, of the inclusion of full Nylon Cap Ply. This would be identified in the material list molded on the tire sidewall along with the number and material in both the sidewall and under the center of the tire tread.

However, the driver] has complete control over the Speed/Load/Inflation factors.

IMO it would be better if all tires had at least a 15% margin on load capacity, with 20 to 25% being better margin. We need to remember that road slant & crown and curves along with significant sideload from wind can easily result in overloading tires that were measured on a level scale. Multi-axle trailers should shoot for a 25% load margin. (read about Interply Shear to understand why )

Ever wonder why RV trailers seem to have a lot of tire problems but your daily driver (car or SUV) doesn't?
Let's look at Reserve Load Capacity of your car:
Many / most have 30% to 40% Reserve Capacity on the tires.

  


Now when we look at an RV trailer we see only 6% with many having 0% margin.
 




 
 If we assume we want to have 25% Reserve load capacity on the above RV, then 13,290/(Tire max load X 4)= 0.75
and each tire max load shall be 4,430 lb.    So the right choice would be an all-steel
ST235/85R16 132/127 (14 with a max load of 4,410 lb.)


Previously I indicated that Inter Ply Shear can be the equivalent of adding 24% more load on the tire while in operation. Using the above tire the IPS effect is reduced considerably.

NOTE in 2017 RVIA and the National Fire Protection Association, guidelines for tire selection for RVs was updated to require a MINIMUM of a 10% margin in tire load capacity over the GAWR. Some RV companies achieved this margin by upgrading their tire load capacity while others simply lowered the GAWR number on the vehicle certification label. Do you know what your RV company did?

   

Speed rating. We should think of this like the engine red-line. Everyone seems to understand that running faster than red line will shorten engine life but that doesn't mean that 5,900 on a 6,000 redline is good to run for hours on end and may contribute to shorter engine life just as running 62 mph on tires rated, based on their original load calculations, for a max of 65 mph.
The high-speed test is a 30-minute step speed test of a brand new tire on a smooth test wheel. No potholes or curb damage. All that is needed to "pass" the test is to meet the target speed and not come apart. The tire is considered scrap after the test.  Would you consider your tires to be "scrap" after you run 70 or 80 mph for an hour or so?
Remember damage (internal cracks) done, even at the molecular level, in tires never repairs itself but only increases in size.

While this post is primarily about Trailer application the limit on high speed also applies to 16 through 22.5" tires that have a max rating of 75 in RV application which would be their "Red Line" but I hear many Class-A owners talking about running over 70 for miles on end but being surprised when they suffer a tire failure.

LT tires would be my first choice in any trailer application where heavy loading was required. I do not understand why so many think that there is some magic engineering used on ST tires that allow them to support 10% to 20% more load than an LT tire of identically dimensions. The load formulas used today for ST type are the same ones used in 1969 when they were limited to 65 MAX (red-line).

I would be very interested to hear an answer from the engineers at the companies that make ST type tires why they were able to suddenly, almost overnight, increase the speed rating from 65 to 75, 87 or even 99 on their ST type tires. Do they have some new materials? Why don't they put those same super materials in LT tires and increase the load capacity of LT tires to be the same as or near to the loads seen claimed for ST type tires?

Sorry, I just don't buy the new claimed high-speed capabilities of ST type tires that exceed the load and speed capabilities of equal size LT tires.

##RVT935

ST tire speed rating

Point of clarification.

The Tire & Rim Association Load formula used by the industry for ST type tires is based on a 65 mph max operating speed. They even identified inflation increase and load capability reductions necessary to operate up to 75 mph.  Neither P or LT tires have stated max speed associated with their load formula or calculation as far as I can find. P-type do have the stated 1.10 "De-Rating" of load capacity when used on a trailer or P/U.

If you look at similar physical sized tires and include the P Derating you will see the de-rated P and LT tires give similar results for load capacity.

BUT the load capacity of the ST type is significantly higher. Why would that be if there were not some other limiting factor such as the 65 mph speed limit?
Some people would have us believe that ST tires all had some magic rubber or body cord added to them in 2017 when speed ratings higher than 65 were added. The reality is that the speed rating was added to avoid import tariffs. Maybe you think that all the different tire companies used this "magic" rubber to give them an overnight increase in speed capability.

Why isn't there a single tire company anywhere in the world using this "pixie-dust" in their LT or heavy truck tires to give them an extra 10% to 25% more load capacity?

I have seen a section of a GY Endurance and it clearly has "better" construction than some non-speed rated tires. Only time will tell if this new construction results in a better life, But I seriously doubt that you can go from 65 mph max to 99 or 106 mph with an ST tire and not suffer some negative consequences.   Yes, many will say they do not drive that fast but we have all been passed by trailers going faster than 75. Some even say they tow at 80+.

Going faster can do structural damage and we have covered how damage is cumulative.

What I have not seen is anyone changing their advice on tire life in RV usage.
What ST tire company now says up to 10 years life or even claims over 5-year life?  I have reviewed a few different owner's manuals and see 3 - 4-year life suggested.
I have previously written about Interply Shear and while the addition of a Nylon cap on top of the belts may extend the life of a tire and it may also improve the tire's ability to pass some speed test designed for passenger tires I still have serious reservations about any trailer being towed at any speed above 65 mph.

I'm not saying "Don't buy tires with higher speed symbol" What I am saying is that you should still stay to 65 mph max or you may end up paying the consiquences with shorter than expected tire life.

But that's just my opinion.
 

Are ST tires the "Best" tires made?

One fact that many choose to ignore, or just don't think about, is that very few RV have the axle load evenly split between axles on trailers or end to end on any one axle. Some owners have learned that there can be upwards of 500# to over 1,000# load unbalance.

It takes more work and effort to learn the reality of YOUR RV tire loading. I and others have covered the how and where to learn the loads on each individual tire (usually not on CAT or similar truck scales).

Regulations are written based on the assumption that RVs have close to perfect weight balance but few owners will make the effort to learn the actual tire loads. Also, those sacred regulations fail to tell the trailer owners that they can expect to need to replace ST tires at 3 to 5 years usage based on the tire DOT serial.  You will see numerous complaints about tire failures but most can be traced to overload/underinflation and over-speed or other external damage.  There is no "magic" rubber in tires with "ST" on the sidewall but many seem to want to believe there is.

As a tire engineer I have yet to have anyone explain why or how ST tires should be expected to perform better than any other tire of the same size on the road, yet that is what some would have you believe based on their insistence that simply because a tire company makes such an ST tire with greater capacity than their premium LT line.  Where is the tire company that makes an ST tire, that will offer a warranty on their ST tires comparable to what they offer on their premium P or LT tires?

Is it OK to run 70 or 80 or faster with ST type tires/ Yes, If you believe in MAGIC

More posts on an RV trailer forum about driving at 70 to 80+.  My reply here....

The load formula that resulted in the numbers in the Load & inflation charts has not changed since the late 60's. That formula was based on a 65 mph MAXIMUM operating speed. The lower speed was the trade-off you get when you increase the load capacity. "THERE IS NO FREE LUNCH"

Look at the load capacity of a similar sized LT tire. If ST tire companies suddenly discovered some magic rubber or special tire design feature why don't they put that magic stuff in the LT and Passenger tires?

The "Speed Symbol" is simply an indication of temperature resistance of a tire on a short-term(30 min) step speed test.   The symbol is based on an SAE test that specifically is for Passenger type tires. ST type is not covered by SAE.

Those who choose to run at 65+ need to remember that you are "consuming" the finite number of cycles that make up a tire life. Faster speed = higher temperature. Higher Temperature = shorter life.


This a Science folks, not magic. So please when you have a belt separation failure before you wear out your tires please do not come back here with some complaint about "Defective" tires of "China Bomb" tires. You were warned but decided to not heed the warning and advice.


The high load capacity in ST tires was developed based on a stated 65 mph Max speed.
IMO The addition of higher speed ratings was not accomplished with improved engineering but because of Import Tariffs being imposed on non-speed rated tires by US Dept of Commerce in 2017 and as if by magic each and every tire company making ST type tires almost overlight added high-speed capability to the tires. There were no years of incremental improvements in speed capability. If that were the case we should have seen so 70 mph tires then 75 etc but we didn't.
 
Some have suggested the increased speed capabilities are the result of years of incremental improvements and that tire companies have seen numerous slight improvements. I understand this concept. The problem is I don't know how to exceed a "Step Speed" test and translate a non-specified step into a speed capability that is not part of the test.

Here are the actual test requirements

FMVSS High-Speed test says

"S7.4   High speed performance. (a) Perform steps (a) through (c) of S7.2.
(b) Apply a force of 88 percent of the maximum load rating marked on the tire (use single maximum load value when the tire is marked with both single and dual maximum loads), and rotate the test wheel at 250 rpm for 2 hours.
(c) Remove the load, allow the tire to cool to 35 °C (95 °F), and then adjust the pressure to that marked on the tire for single tire use.
(d) Reapply the same load, and without interruption or readjustment of inflation pressure, rotate the test wheel at 375 rpm for 30 minutes, then at 400 rpm for 30 minutes, and then at 425 rpm for 30 minutes.
(e) Immediately after running the tire the required time, measure the tire inflation pressure. Remove the tire from the model rim assembly, and inspect the tire."

The RPM converts to 50mph, 75 mph, 80 mph & 85 mph so the steps are not linear.  What would you consider exceeding the specified test by 15% look like?  run 34.5 minutes at 85  or run another 30-minute step at 97.75 mph?

The Speed test above is required for a variety of tires with ST being one type. LT and Pass tires have different standards and different test requirements.

None of the DOT tests support any "Speed Symbol" claim as DOT doesn't recognize the speed symbol.
SAE high-speed test is not a regulatory test.

J1561_200102
"This SAE Recommended Practice provides a method for testing the speed performance of passenger car tires under controlled conditions in the laboratory on a test wheel. This procedure applies to “standard load,” “extra load,” and “T-type high-pressure temporary-use spare” passenger tires."

Maybe someone can explain how a P235/75R15 would be rated for 1,874# @ 32 psi and can carry an "S" speed symbol (112) but in trailer or pick-up service is only rated to carry 1,703#
An LT225/75R15 is rated for 1,445# @ 35 psi and can carry an "S" speed symbol
Then we see an ST225/75R15 is rated for 1,760# at 35 psi with a Speed Symbol of "N" (87)

If you only want to look at dimensions, load & inflation I don't know why all three wouldn't be rated for 1,703 to 1,760# load capacity.
 Why haven't the tire companies put the same better construction features they apparently are putting in the ST tires in the LT tires? where load capacity is an important factor in selecting the vehicle?

##RVT866

Are ST type tires "Required" on trailers?

Had a question:
"My 2012 Airstream Classic came with Goodyear Marathon ST225/75R15 LR-D tires. I would like to tell you how many original installed tire I have had blowout using the original and same size tires as replacements. I even replaced the LR-D's with LR-E's and still only get about 3,000 miles before blowouts. It is the damage caused by the blowouts that concerns me most. I am tired of fixing my trailer after blowouts.

As I alluded to in my earlier message, I owned an Alpha 5th wheel that had 16" wheels as the original size. After 5 blowouts in one 6,000 mile trip I replaced the tires with Michelin LT235/75R16 XPS RIB (All Steel) tires. These tires are made with steel sidewalls. Many thousands of miles later I traded the trailer for my present Airstream without any more blowouts on my Alpha.

I have another question that does not appear to have a good answer....If trailer tires are so good and must be installed on a trailer when truck tires wear so much better, what is the rational for requiring the ST's on a trailer? My truck, 2002 GMC Duallie uses LT tires and I can not wear them out not blow them out before I feel it is safe to replace them (7 years). I am running LR-E' LT types on my truck.

I am sorry to say that I do not agree with the idea that ST tires for trailers is the safest and best option.

I have never seen any "requirement" that trailers run ST type tires. However, ST type tires can only be installed on trailers, so that might be the "requirement" you are thinking of.

The selection of tires to use on any vehicle is up to the RV company. As far as I can see they select tires based only on load capacity, physical size and cost. I don't know what the RV company actually pays for ST tires but it is probably less than an LT of equivalent load capacity.

If your only concern was for lower cost when comparing two tires of equal load capacity and the ST tire was easier to fit (it was smaller and did not require you make larger wheel wells), Why would you select the higher cost tire?  If you don't offer a meaningful warranty, you do not face the expense of warranty costs if there is a tire failure so what is the incentive for the RV company to spend more?

Selecting alternate or replacement tires for large / heavy trailers

Found a thread on a forum for folks who own large heavy 5th wheel trailers. This info would apply to non-5'ers too.

While I understand the concern for the tire dimensions, that is NOT the most important specification.
Number one is to ensure any replacement tire is capable of supporting the load you are placing on your tires plus a margin.

The best thing to do is to first confirm your actual tire loading.
Ideally, you would get on a scale, with the RV loaded to the heaviest you ever expect to travel with, and learn the actual load on each tire as there are very few RVs with the load split evenly axle to axle or side to side.
HERE is a worksheet you can use.  You will have to do some hunting around as you can't get individual loading on most truck stop or CAT scales. You will need to find a local building supply or feed or grain dealer or gravel pit or possibly cement delivery company.

Lacking that you could use a truck scale but to be safe you need to apply some math to estimate the load unbalances.
First, assume a split of 52/48 between axles or with a three axle trailer assume one axle is supporting 35% of the total. Then assume a 53/47% split on the heavier axle for side to side loading.  Yes, some RVs have been measured with individual position scales and found 1,000# un-balance.

So with the measured or calculated heaviest loaded tire, and the dimensions checked, you are ready to shop for tires.

You need to realize that ST tires have a higher load capacity than LT type tires. This is because the load formula for ST type tires is based on a max speed of 65 mph even if the "Handling rating" speed symbol suggests differently. So you can't just use the numbers when comparing tire sizes as an ST235/75R16 carries significantly different load than an LT235/75R16  even with the same Load Range ( D or E or F etc)

You can then consult the Load & Inflation tables for the tires under consideration. The good news is that with the exception of Michelin 99+% of the tires out there follow the same table info so you can use Bridgestone or Goodyear etc for LT and Maxxis or Goodyear for ST type tires.. You can look at different tables HERE if you want.

When selecting a tire you need to get the tire capacity at least 15% greater than for your measured or calculated tire load. This allows for sway, load shift due to road crown and wind side load to the tires you are buying.

After you do the above THEN you can confirm tire dimensions knowing the load capacity needed.

On my blog, I cover why you should run the inflation number molded on the tire sidewall (lower the Interply Shear) and why you should always run a TPMS along with other info on Interply Shear and the effect of temperature on tire pressure.  You might even subscribe.

Hope this helps.

ST type tire speed ratings. One man's opinion.

Lets see if I can bring some Science and Engineering facts and history to this issue of speed limits on ST tires.

In '60's & '70 when ST type tires were "invented" and started to be applied to Travel Trailers, the national speed limit was 55 mph and tires were bias.  Trailers were considered "big" if they were 24' long and I doubt there were many if any 5th wheel tri axle trailers on the road. 

Today we see speeds across the country of 70+ and there are many locations where you could set the cruse at 70 and never slow down for an 8 hour drive here in the US. Trailers over 30' are normal with some pushing 40 feet and most have tandem axles with more tripples showing up every day.

The formula for determining the load capacity for all tires follows the basic format 

  Load = K  x  (air pressure)   x  (air volume)

Now the calculation for air volume is the complex part as aspect ratio and a theoretical rim width and other factors such as tread depth come into it but these details do not change the fundamentsl format of the formula.

The "K" shown above is an important concept as it is really a factor based on the expected service.

Standard passenger cars seldom if ever carry their max load. The GVWR and GAWR are not even in every owner's manual or on the Vehicle Certification label AKA "Tire Placard". They are expected to be run at posted speeds but on paved roads for hours on end and driven 10,000 to 20,000 miles a year  i.e. used fairly frequently with many being parked in a garage.

For the sake of this discussion lets assume the K is set to 1.0 for passenger cars.

Now what do you do with Station Wagons and other "multi-purpose" vehicles? These vehicles were expected to carry more load more often so the service is obviously more severe.  When SUV's came along they were places in the "Multi-purpose catigory" and if a passenger type tire was applied to a trailer that was also considered more severe service. So the load capacity was reduced. many are aware of the "De-rating of P type tires when used on trailers or SUVs etc. So K (multi-purpose) = K (passenger) divided by 1.10 and we end up with lower load capacity. About 90% of passenger.

Lets look at the actual numbers.

P235/75R15 105S  35 psi

  2,028# @ 35 psi 112 mph on a Passenger vehicle

  1,844# @ 35 psi 112 mph on an SUV or P/U or trailer

Moving on to Pick-up service we have LT type tires. The formula is still K x pressure x air volume but with trucks expected to carry even more load most of the time their K factor is different.

Their numbers give us

LT235/75R15 101/104Q  LR-C

  1,985# single 50 psi 99 mph 

This lower load capacity on truck service is clearly because the higher percentage time spent carrying more load.

Before we move on lets look at the ST numbers

ST 235/75R15  LR-C

  2340# @ 50 psi   65 mph

To me the obvious question should be: How does the addition of the letters "ST" on the sidewall allow a 26% increase in load capacity over a P type tire (adjusted for trailer service)

or a 29%  increase over the heavily loaded but occasionally empty truck? The only reason I can see is the significant reduction in speed.

We all know, or should know that more load (more deflection or bending) generates more heat so what could you do to counteract the increase in heat due to the increase in load? Obviously lower the speed would reduce the higher heat and that was part of the original ST tire standard.

Now lets look at the tire type that is of real interest. ST type as used on many RV trailers. 

In 2014 new duties were imposed on imported tires but ST type were exempt, sort of. There were various requirements some of which were requested to be changed or eliminated. The speed symbol was one of these requirements.

Starting in 2017 (possibly earlier in small quantities) many ST type tires started showing up with a Speed Symbol selected from the table as published by US Tire & Rim Association in the LT section.

The problem is that Speed Symbol does not have any standard DOT test or requirements as in the US Speed Rating is really a marketing tool and not a strict performance requirement. A review of various ST tires shows a range of speed symbols from L (75 mph) to R (106 mph) and possibly higher.

Further compounding the confusion is that the speed symbols are from the SAE - Society of Automotive Engineers and according to SAE their test criteria J1561 apply to ""standard load," "extra load," and "T-type high-pressure temporary-use spare" passenger tires." This raises the obvious question of what test procedure, if any, are various tire companies following when they assign the Speed Symbol? While we are talking about SAE symbols we need to remember that DT does not recognise or test for these ratings.

Let me close with a question I have asked a number of times but as of now have never received an answer for.

What "magic" engineering are tire companies puting in their ST tires that allows them to run 75 or 81 or even 106 mph without making any adjustments in load or inflation? and If they have this "magic" engineering available, why aren't they using it in their LT tires? There are construction features that will improve the test performance of tires, maybe even improve the speed rating a step, but I have serious questions on the ability of tire company to increase load capacity by 20 to 25% which is effectively what some appear to be trying to do.

NOTE Goodyear Tire Care Guide (https://www.goodyearrvtires.com/pdfs/tire-care-guide.pdf) clearly shows a blanket 75 mph max speed for 17.5 rim diameter and larger tires.

Wonder why they didn't increase the speed rating of these tires using similar construction features they applied to the GYMarathon to turn it into a GY Endurance.

Tire info in Owner's manual and on Certification label

I have been looking at the "Tire Placard" AKA Vehicle Certification Label for a number of 5th wheel trailers. I learned a few things.

A couple of manufacturers have upgraded the tires on the newer units. Some went from ST235/85R16 LR-E to  ST235/85R16 LR-F. The inflation went from 80 to 110 so they are clearly offering improved load capacity or giving you a better margin above the tire capacity.

If you have an older 5th wheel and have either ST235/80R16 LR-E or ST235/85R16 LR-E you might want to check and confirm if your wheels are rated for 110 psi. If so, you might consider moving on up to LR-F and doing the slight up-size if you have the needed clearance as the 85 series tire is a bit larger in OD.

I also found one model that went from 16" to 17.5" wheels. It appears that the new RVIA 10% load margin requirement achieved some better than expected results. While this may not help out those with older trailers just knowing there are wheel & tire options available that are approved by your RV company could be considered good news.


On a related matter, I wonder how many of you have ever read the section on Tires in your owner's manual. I was surprised to see that some manuals have pretty good and detailed info. They cover stuff like weighing each side of the RV and not just the total on the 4 or 6 tires. While this isn't as good as learning the individual load on each tire a side to side comparison may identify some load imbalance. Since I suggest that trailers run the tire sidewall inflation we are not looking at calculating a minimum inflation. BUT we still want to be sure you have at least a 15% margin on load, so you should use the heavier side of the RV when doing your"Safety margin" calculations.

I also found some clear info on "cold inflation" and operation speed which was better than I expected. Tire life and expected replacement age were not uniform brand to brand but some of you may be surprised to see that some manufacturers actually tell you the number of years use you should expect from your tires.


Hope to meet a few of you at FMCA GLAMARAMA in Michigan in a couple weeks or in Gillette, Wy in July.   Safe travels.

Changing tires on a trailer - NEW load capacity requirement

On many of the RV forums I monitor that focus on trailer application, there is a recurring question about changing tires. Some wonder about going up in Load Range (Ply Rating) some wonder about changing the "Type" tire P > ST, or P> LT, or ST > LT. Others want to change the tire dimensions. While there are many reply posts, I do note that not everyone offering answers has worked as a tire design engineer. It takes years of working with the engineering and scientific knowledge before you can be given the responsiblity to develop a new tire capable of passing various company and DOT regulations and be produced for sale in the tens or hundreds of thousands.

While I have tried to provide answers, I seem to end up saying the same things over and over so this post is intended to be a go-to post for those asking tire change questions.

First, it is important that the owner know the ORIGINAL tire size including the Type and Load Range and the recommended inflation from the RV company along with the GAWR for their specific Trailer. Finally if considering a change we need to ensure that the new tires can properly support the ACTUAL load on the new tires.

I will start off assuming the owner is keeping the tire dimensions the same i.e. 225/75R15 > 225/75R15. Note I said "dimensions" not the "size" as a tire engineer "size" to me includes the Type + Dimensions + Load Range

For P > ST or P > LT you need to remember that application of a P-type tire on trailer required that the RV company "De-rate" the load marked on the tire sidewall   Sidewall/1.10 = load capacity of a P-type tire on a trailer.
For ST > LT you will probably need to increase the dimensions and or Load Range to achieve sufficient load capacity.

The general rule of thumb "Any replacement tire MUST be capable of supporting equal or greater load than the original tire".
Another rule: You need to ensure that any tire you use is capable of supporting your actual MEASURED load, Not the load your neighbor said he has and not an estimate or the measured load someone posted on a forum. The load measurement ideally should be obtained with your trailer at its heaviest i.e. fuel, water, propane, clothes etc  If you can't get individual one side weights DO NOT assume a 50/50% side to side load split. While some trailers may be balanced at 49/51% but some have been found as much as 10% off balance i.e. 40/60% As a rule of thumb I suggest you use at least an assumed 47/53% splits you would use the 53% figure.
If making an investment in new size tires & wheels you really need to learn the real loads before making the change or you may discover you bought tires you should not be using.


You will need to consult the published Load & Inflation tables for your old and new tire to confirm load capacity numbers. I have THIS post with links to many different tire companies. Be sure you understand how to read the tables as while most provide load capacity per tire, some load figures are per axle. DO NOT use the "Dual" load numbers as these only apply when there are two tires mounted as a pair on each end of an axle.

Comment on valves. I always recommend that whenever changing tires, even if you are just replacing with same size and type, that rubber valves be replaced with bolt on metal valves and if you already have metal bolt in valves that you get the various rubber gaskets and "O" rings replaced as these rubber parts age out just as tires age out and it's awful to read about a $2 valve failing which can result in hundreds or thousands of dollars in damage and costs.

Finally, some new info that all RV owners should consider:
Late last year RVIA updated the tire type and load spec such that "based on the rating of the axle the tires have to be 10% greater than the axle rating,"  You will note that RVIA decided to ignore the reality of load unbalance.
Clearly, if you are getting new tires it makes sense to incorporate this new safety margin in your calculations.

I want to thank my fellow RV owner and tire design engineer CapriRacer for doing a bit of technical editing on this post. He also has a blog on tires. 

Next week I will do a post on trailer tire inflation.

If you find this post helpful and happen to see someone posting questions about changing tires please consider posting a link to this post as I don't see every tire question posted by all RV owners.

ST type tire max speed

Originally Posted by LI Pets  The 65mph is not really correct it depends on load and air pressure. They can go to 80mph.

The above is from an online forum.

As I have pointed out previously:

The load formula that populates the Load/Inflation table, used by the tire industry for ST type tires is based on the assumption that 65 mph would be the MAXIMUM operation speed. If the operation speed was to be between 65 and 75 then users were to use a 10 psi higher value for their measured load.

Example ST235/75R15 with 2,030# measured load needs to run 50 psi minimum cold not 40 psi cold.

If running 75mph to 85 the Load capacity number must be reduced by 10% in addition to the +10 psi adjustment.

Example ST235/75R15 would be rated to carry (2,030 x.9) or 1,827# at 50 psi.

I know this adjustment process seems backward but that is the way tire loading is calculated.

Now I am sure some will say that the new ST tires come with speed ratings faster than 65. IMO many of these ratings were applied just to avoid import taxes. I know of no magic rubber that somehow gives an ST235/75R15 LR-C the ability to support 2,340# at 87 mph with 50 psi in it while an identical sized LT235/75R15 LR-C is only able to support 1,985#.

Before you say "Ya but the tire companies probably made big improvements in the new ST tires", I would ask what makes you think the tire companies would not want to be able to offer better load capacity to their LT tires?

Load capability is basically the tire air volume x air pressure, with adjustments for speed and expected service. So if you have a P/U pulling a TT the "service" would be the same and the speed would be the same so how can the tire with "ST" on the sidewall carry more load at the same speed?

Magic?


Now you are more than welcome to believe in magic or marketing claims but IMO using the load /infl tables without doing the adjustments will probably result in an increased likelihood of belt separation. So when you have a failure please do not post something here along the lines of  "I just had a blowout. I didn't hit anything and always check my air".  Tire failures usually occur because of cumulative internal structural damage from heat and time. The excess heat comes from the combination of speed/inflation/load.

What effect does speed have on tire failure?

Question:
"What effect does speed have on tire failure?

I have an 8000 lb trailer with 4 Goodyear Marathons. The truck is a Dodge 2500 diesel so I can cruise at 75 mph subject to wind and hills.

How much if any does speed factor into tire failure? I replace tires every 4-6 years regardless of what the look like, and keep them inflated at the PSI marked on the tires."

Basically, increased speed means increased temperature in the critical areas of a tire. Here is a graphic showing the relative temperature in different areas of a tire.

with Red being the hottest and dark blue being the coolest.

When you increase the temperature of rubber you are effectively increasing the rate the rubber is aging. I believe we all understand that old rubber is not as flexible as new rubber. If it isn't as flexible you end up with cracks or cracks that grow every time the rubber is flexed. More cracks and larger cracks are what result in tire failure.

For the above reasons plus others, the major tire companies suggest that in RV operation tires never be driven any faster than 75 and you can see this max speed stated in their literature for many of their RV tires.

But wait, you say your tires have a "Speed Symbol" that relates to 87 or 99 or maybe even 130. HERE is some information on speed ratings. Just because you have a tire with a fast symbol doesn't mean it can be driven at those speeds for 20 or 40,000 miles. About the only thing you can count on is that tires with a "higher" symbol will provide better steering response than a tire with a lower rating.

Now back to the question of how fast can you drive or tow your RV. I previously mentioned 75 as the upper limit but for some tires, there are other things to consider. With ST type tires as found on many trailers, we need to remember that the load capacity is based on a formula that originally specified a max speed of 65 mph. In the past, there were Technical Bulletins that advised that inflation needed to be increased (but not above the sidewall max) by 10 psi to go 70 mph. In addition, load needed to be decreased by 10% if you want to go 75. Still, there was that 75 max.

Many trailer owners know that they need to replace their tires before they wear out. If they don't they are probably going to have some type of failure. But those same people will not replace the tires on their pick-up till they are significantly worn. Why?

The answer can be seen if you simply compare the load and speed ratings of an LT tire and an identical size ST type tire.

LT235/75R15 101Q LRC1985# 50 psi 99mph

ST235/75R15 110R LRC2340# 50 psi 106mph

So exactly what type of magical engineering has tire company X used to achieve both increased load capacity plus increased speed capacity when it is the air pressure that supports the load? If they have the ability to make that ST tire really perform at those speed and load conditions what is wrong with their LT type tires?

IMO what we are looking at is a marketing plan taking over the engineering reality.

Bottom line

You may be able to pull the trailer at speeds above 65 or drive the Class-A diesel pusher faster than 75 but you will be consuming the finite life of the tire and can expect a failure before you wear it out no matter how you maintain your tires.

##RVT817

How fast can an ST tire run?

Originally Posted by SNOKING

So Roger, can you explain how Carlisle has raised the max speed of their ST tires like the Radial Trail RH? Chris

It's easy. Just run a drum test for a few minutes and you can claim the tire "passes" the requirement for the speed symbol.

My point is that the fundamental basis for the load formula was developed back in the '60s & '70s when the National Speed Limit was 55 mph. To my knowledge there have not been any substantive changes to the formula since then, even though highway speeds are now much faster than 55. It is also important to remember that in 2002 other tires like LT and P type had a major overhaul of the regulatory requirements to make radials more robust, but it is my understanding that the RV industry opposed the application of stricter, modern requirements be applied to ST type tires.

DOT regulatory test requirements for ST tires are over 40 years old and the only requirements for "High Speed" are as follows from FMVSS 571.109:

"S5.5 High speed performance.

S5.5.1 After preparing the tire in accordance with S5.4.1, mount the tire and wheel assembly in accordance with S5.4.2.1, and press it against the test wheel with a load of 88 percent of the tire's maximum load rating as marked on the tire sidewall.

S5.5.2 Break in the tire by running it for 2 hours at 80 km/h (50 mph).

S5.5.3 Allow to cool to 38° ±3 °C (100° ±5 °F) and readjust the inflation pressure to the applicable pressure specified in Table II.

S5.5.4 Without readjusting inflation pressure, test at 121 km/h (75 mph) for 30 minutes, 129 km/h (80 mph) for 30 minutes, and 137 km/h (85 mph) for 30 minutes."

NOTE the above applies to a new tire only and the inflation in "Table II" is 2 psi below the max on the tire sidewall. Once a tire completes the above without visible failure it is considered to have "passed" the test. You will note the relative short cumulative time and that the test is on a new tire that has never hit a pot hole or curb, or been exposed to sunlight for weeks, months or years.

SOURCE Here

LT tires must meet the following under 571.139
"S6.2.1.2 Test procedure.

S6.2.1.2.1 Press the assembly against the outer face of a test drum with a diameter of 1.70 m ±1%.
S6.2.1.2.2 Apply to the test axle a load equal to 85% of the tire's maximum load carrying capacity.
S6.2.1.2.3 Break-in the tire by running it for 2 hours at 80 km/h.
S6.2.1.2.4 Allow tire to cool to 38°C and readjust inflation pressure to applicable pressure in 6.2.1.1.1 immediately before the test.

S6.2.1.2.5 Throughout the test, the inflation pressure is not corrected and the test load is maintained at the value applied in S6.2.1.2.2.
S6.2.1.2.6 During the test, the ambient temperature, measured at a distance of not less than 150 mm and not more than 1 m from the tire, is maintained at not less than 32°C or more than 38°C.
S6.2.1.2.7 The test is conducted, continuously and uninterrupted, for ninety minutes through three thirty-minute consecutive test stages at the following speeds: 140 (86mph), 150 (93 mph), and 160 (99 mph) km/h.

S6.2.1.2.8 Allow the tire to cool for between 15 minutes and 25 minutes. Measure its inflation pressure. Then, deflate the tire, remove it from the test rim, and inspect it for the conditions specified in S6.2.2(a).
S6.2.2 Performance requirements. When the tire is tested in accordance with S6.2.1:
(a) There shall be no visual evidence of tread, sidewall, ply, cord, innerliner, belt or bead separation, chunking, open splices, cracking, or broken cords.
(b) The tire pressure, when measured at any time between 15 minutes and 25 minutes after the end of the test, shall not be less than 95% of the initial pressure specified in S6.2.1.1.1."

Source here.

Note the test pressure in 6.2.1.1.1 is about 5 psi below the max on the tire sidewall.

Also remember that an LT225/75R15 LR-D carries 15% less load than an identically sized ST type tire. I would ask what part of physics allows an ST to carry more load at the same inflation as an LT if all other conditions of use are identical?? Just making the container stronger doesn't result in increased load capacity even if it was the container that supports the load, WHICH IT IS NOT. It's the air pressure that supports the load else why have "Load & Inflation" tables. If it was the tire weight or mass we would have tire weight vs load tables.

If you want to learn more about the speed rating test tire companies are using I suggest you read This post written by another tire engineer.

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##RVT791

Question of running year old tires or Running less than max psi

Question from Chris

Hi Roger,

I came across your website by way of referral from a member on a Keystone RV forum.  I have a concern regarding upgraded tires and wheels for a Keystone trailer I own.  

As most manufacturers continue to cut costs in value models, wheels and tires seem to be ground zero in their savings.  So, to help mitigate tire failure on my coach, I've upgraded my china bomb OEM 14" tires sized ST205/75R14 LR-C to ST225/75R15  LR ? and purchased new wheels to accommodate the new rubber.  My wheels are Sendel wheels that support 2540 lbs of load each. 

I purchased Carlisle Radial HD tires in LRD from Discount Tire.  Unfortunately, they received LRE tires in instead of the LRD tires.  The Discount Tire salesman quickly resolved the error and within 2 days, had LRD in stock.  I returned to the store to have them balanced and mounted on the wheels. The stock they received in had DOT date code of 1516. He told me that was the newest in all his warehouses. I didn't want year old tires so I rejected the sale.  

Then, I ordered the same tires from Walmart.com.  Same story, the DOT date code was 1616 and 2016.  Rejected that sale.  Finally, I ordered them from Amazon.com.  They arrived today.  Again, tires are around 1 year old and I'm sending them back.

My first attempt at ordering tires from Discount Tire resulted in LRE tires. They were much newer around Nov 2016. I think my quest for load rated D tires from Carlisle in size 225/75/15 is futile.

So now, I'm considering purchasing the LRE tires as I think the local stock is much more fresh than I can ever find with LRD. The only limit I have is my wheel capacity at 2540 lbs. If the LRE tire max inflation pressure is 80 lbs supporting 2830 lbs, will it compromise the tire if I inflate only to 65 lbs (in order to stay within the specs of the wheel)? The OEM tires were LRC on 14 inch wheels supporting only 1760 lbs at 50 lbs. Weight is not a concern as I will never come close to max weight limits with my trailer on either LRD or LRE tire. 

Another thing that worries me is the stiffness in the LRE rubber. Comparing the Carlisle LRD to the OEM Trailer Kings is night and day. The tire is just so much more beefy. Don't really want to rip apart the trailer because it's rolling on unforgiving rubber. 

Any opinions and recommendations is greatly appreciated.

Thanks in advance.

Chris M.

Concerned RV owner

+++++++

My answer:

OK you have a couple different issues.

You need to know that tires are made in batches. For relatively small markets it's possible that a company may only make an item once a year or even less frequently. While tire age is of concern when the tires are on the ground and exposed to sunlight and heat, it's less of an issue when in the factory warehouse.

One way to let your mind rest a bit is to ask when the warranty clock starts ticking. Some companies clearly state on day of application or first retail sale. Others say the warranty is based on DOT serial. If it's based on date of sale and clearly stated as such in the literature, I would be less concerned and just be sure that the sales receipt has the date and DOT serial of each tire identified and keep both warranty and receipt in a safe place.

Now, since you have taken the step of going from the ST205/75R14 LRC to ST225/75R15 LRD, you have increased the load capacity from 1760 @ 50psi to  2540 @ 65 psi. You have increased the reserve load capacity by over 50%, which should deliver very reliable service. While I do not have any data to go on, I think that when inflated to the same psi the LR-D and LR-E would have similar stiffness. It's the air that supports the load and it's the air that provides the majority of the tire stiffness.

For your application I would think there would be no problems with running LR-E tires at the wheel limit of 65psi.

So, bottom line, I think you can go with the year-old LR-D tires or run the LR-E at 65.

Change Tire size

When considering a change in tire size there are many different areas you should consider.

Tire load capacity. You need to be sure you go with tires that have equal or greater load capacity than original.
It is possible that the original tires delivered a very small load capacity such that the coach is easily overloaded. Recalls may have been ordered or initiated but sometimes a manufacturer went bankrupt so no recall action took place or no new tire certification label was provided.
So it is up to you, the current owner to do your homework and get the facts.

- You need to learn the real load capability of the axle by contacting the manufacturer if there is no plate on the axle with GAWR info.
- You also need to learn the real loads you are placing on your tires with your coach fully loaded. Here is a worksheet with instructions on the steps and calculations you need to do.

Now load capacity is not the only information you need. You also have to be sure the tire will never rub and contact any portion of the coach or its suspension. part of this is obvious and can be discovered by rolling on your back with a flashlight to confirm sufficient clearance around all your tires. Don't forget to check both front tires with wheels turned completely both directions. I have read that 3" on all sides is a reasonable clearance.

Rim width - There is a list of approved widths for each tire size. You must stay with the dimensions listed by the tire company

One final clearance check that is sometimes overlooked is the clearance between dual tires. This requires more than just looking at the rear tires. In tire company specification charts there will be a dimension called "Minimum Dual Spacing" for the tires you may be considering. This dimension is controlled by the wheels. Sometimes this information may be marked on the wheel but most likely you will need to contact the wheel manufacturer to learn the specification. This is CRITICAL, as too small a clearance can result in tire damage or even failure.


I can't address if a specific size will fit but simply going up one size would be reasonably close to the 1/2" width increase.

For example if your current size was an ST215/75R15 going to run ST225/75R15 would give you a tire that was approx 10mm ( about 3/8" wider over all)
Now it is important to remember that tire dimensions are approximate and if you change brands the actual with may be a little different.

Also don't forget that the OD would also increase which means the clearance between the tread and wheel well or the companion tire would change also.

You should be able to find published OD and width dimensions from your tire companies web page.

One other item is that a change in wheel width (as published in the literature) will affect the tire width by about 40%. So a tire with a published width of 10" on a 6" wide rim would be about 10.2" on a 6-1/2" wide rim (40% of the 1/2" rim change = about 0.2")


 But in some cases on older coaches tires may have been changed in size or even the size designation such as an old 7.50-15.  If you find yourself in that situation you might consider dropping me an email so I can help walk you through the things you need to consider.



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How you might run ST type tires at 75 to 80

RE Speed Ratings and operation speed.

My comparison to engine redline seems to me to be the easiest for many to understand. It is possible to run an engine right at redline or even above for a short time but I can't find anyone that advocates operating an engine at the rated max speed for any significant time or distance.

High Speed test is not a DOT regulatory test but is a test based on Society of Automotive Engineers testing. To be marked with a speed symbol a new tire needs to run for 30 minutes on a smooth drum at the stated speed.

There is no requirement for a tire to be "conditioned" with a few thousand miles at lower speed over potholes and up on curbs or with 110% of the rated load or for the tire to be able to pass DOT regulatory durability tests after running the SAE High Speed test.

Yes, an increasing number of ST type tires now come with a speed symbol molded on the sidewall. The primary reason for this seems to be to avoid import duties. What many want to ignore is the fundamental truth that the load capacity of ST tires is much higher than an LT type tire based on the premise from 1970 that the ST tire would be on a single axle trailer that was limited to 50 mph operation speed.

Molding the letters "ST" on a tire is not magic. Physics still applies. If people want to drive at 75 or 80 while towing as they would with their LT type tires and not have failures, then I suggest they pay attention to the Physics and limit the actual tire load as if it were an LT type. This is easy to do.

First simply look for an LT type tire with the same dimensions i.e. ST235/75R15 > LT235/75R15 in the Load tables and limit their measured load to the number found in the LT tables.

If you do that you will probably see a significant reduction in tire failures. Of course this also means you are not in the 50+% of RV owners that operate your tires under-inflated.

If you ignore the Facts and Physics of reality you will have to live with the consequences no matter how much you want to believe otherwise.

Bottom Line. If you want durability and life more like LY type tires then treat them as if they were LT type tires.

IMPORTANT NOTE and WARNING
As with the engine in your RV or tow vehicle it may be possible to run faster but it does not mean you will avoid all problems. This post is just about tires and not about safe and reasonable operation of your RV. Personally I think 70 is too fast to drive a "big rig" and 75 is certainly too fast for towing. I have heard comments about an increase in truck tire failures due to increased speed limits in many states. A MAX of 75 is stated in some tire information guides published by major tire companies and as with any maximum, the closer you are to it the more likely you will have some negative consequences.


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##rvt750

Which brand tire is better ? I can set up a test if you really want the answer.

Many people ask the question in various RV forums "I need new tires. What is the best brand tire I can get?" or "Why do tires made by company Q seem to have a high a failure rate?"

Invariably the thread quickly devolves into people providing their opinion but I do not recall ever seen the suggestion that one brand is better than another supported with any data based on direct apples to apples comparison of two brands of tires.

I believe there are two reasons for not getting a clear picture of which tires provide better service in RV application: One reason is personal pride and the other is a lack of direct comparison based on equivalent exposure to service problems.

Personal Pride
I think you will find that many posts IMO are based on the concept that people do not want to consider the possibility that the product they bought isn't the best. To think so could be an indication that someone might not be making the best purchasing decisions. For example, if someone asks about Acme brand anvils, and someone just happened to have purchased an Acme anvil they are more than willing to "badmouth" anvils made by almost any competitor even when they lack either meaningful data or solid experience to justify their position. After all why would they admit to just having purchased a good quality anvil?

Direct comparison of tire performance.

You may not be interested in ST type tire but I would hope that if you review this information it may help you make a more informed decision when asking what brand to buy.

If you consider that the OE market in ST type tires in RV trailer application is dominated by Goodyear and many dozen 3rd or 4th tier tire companies who make tires in small lots and don't even apply the name of the tire company to the tires they are making. With GY having a majority of the market and no other brand having much more than 5% I think you can see how it easy to end up with skewed impression that GY tires are bad, based simply on the observation that the greatest number of complaints are about Goodyear ST type tires. This is much like the concept that tires made in China must be poor quality because so many trailer owners that have problems with their tires discover they are made in China. I have to wonder how people expect to be able to make a reasonable comparison of tire quality based on country of origin when probably 90+% of RV trailers with ST type tires come OE with tires made in China. Wouldn't it seem reasonable then that 90+% of problems experienced on RV trailers with ST type tires would have tires that were made in China?

Now in 22.5 rim diameter tires found on Class-A RVs,  I think we see Michelins with sidewall cracking complaints and Goodyears with irregular wear complaints and since those two companies probably are on 70+% of coaches of course those are the ones that would have the most and therefore most noticeable complaints. Even if another company with maybe 5% of the market had tire failures  at twice the rate of GY or MI cosmetic issues, it would still appear that GY and MI are the only tires with problems.

Check out my post on Causation vs Correlation for more on the topic.

Thorough tire testing costs in the neighborhood of $30k to over $100k per brand for just basic outdoor wear and general durability is it any wonder that no independent agency has stepped up and run such a comparison?

Now I have the contacts in the tire industry to an independent testing company that could run such a comparison. All I need is for people to send me contributions - maybe in $500 increments and I will develop the specifications for a direct comparison between Goodyear ST type tires and Maxxis brand ST type tires plus a set of TowMax or similar ST type tires. I would use a popular size and load range made by all three companies. To keep costs down we would limit the test duration to about 10,000 miles or till there are enough failures to discontinue running that brand. I figure two failures would be enough to stop running a given brand.

So do you really want to see actual data? All I need is $35,000 in the bank to start the evaluation. Test results would be sent only to those making the minimum $350 contribution or more.
If interested send me an email and I can send you specific details of this offer. We only need 100 individual contributions to get started.


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How to recognize what might be a better tire to consider buying for your trailer application?

Something to consider when shopping for tires for your trailer:

- Many tires have essentially no warranty, others maybe 12 months and some for years. In my opinion (IMO) the length of time the tire company is willing to stand behind their products is one way to judge the real quality of what they make. Which would you consider a better deal: a hammer with a 30-day warranty for $5, or one with a 10-year warranty for $10?

- Speed rating molded on the tire sidewall to me indicates the manufacturer has made a real effort to exceed the minimum performance requirements from DOT. IMO, tires with a speed symbol are probably better than those without a speed rating.

- Here is a chart to convert the speed symbol letter to a number.

- The speed symbol letter is part of the service description which comes right after the rim diameter in the tire size. Technically, an "LT235/75R15 107/110 Q Load Range D"  is a different tire than an "LT235/75R15 Load Range D" -- with the 107/110 being a load index for dual and single application and the letter Q being the speed symbol.

- I note that most LT tires made today have a speed rating, but when applied to trailer application, some companies say to consider 75 mph as a max.

- I also have recently seen a few ST-type tires now being manufactured with a speed symbol which IMO would make them probably a better choice than tires without a speed symbol.


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New Speed info for trailer tires

Learned about some interesting new developments that involve speed limit ratings for ST type tires as used on a majority of RV trailers.

On Nov 24 and updated Dec 30 the US Dept of Commerce issued preliminary guidelines as they pertain to imported tires. This involves what are called "countervailing duties" (CVD) These anti-subsidy duties, are trade import duties imposed under World Trade Organization (WTO) rules to neutralize the negative effects of subsidies from foreign governments that benefit the production of goods from foreign companies.

Now without going into all the legal details this means that some import tires will have a fee added to their price unless they receive an exemption. The fee can be in the 12 to 81% range, so these are meaningful.

This new fee will apply to P and LT type tires primarily made in China. ST type tires will receive an exemption but in addition to the ST as part of their size designation the tires must have either
“For Trailer Service Only” or “For Trailer Use Only” molded on the tire sidewall.

Now the Nov statement indicated that ST type tires must also have a Max Speed in MPH or a Speed Symbol letter also molded on the tires. I thought this was great news as it would then become obvious to RV owners that they needed to limit their towing speed.

However the Dec "clarification" excluded the speed limit requirement.

BUT

I have also learned that a few tire importers have gone ahead and will probably be marking their ST type tires with either "Max Speed xx MPH" or with the Speed Rating letter symbol. The information I have is that the symbol would probably be "M" (81 MPH)  or possibly an "N" (87 MPH) and that tires might be available by early this Summer with these speed ratings.

Of course if a tire company is moving from a tire rated for 65 mph as current ST type tires are, to a higher speed they will need to make some changes in the actual tire construction.

While the final rule has not been published, I wanted to let people know that there may be some short term confusion as both tires, literature and web pages need to be updated to reflect what the tire capabilities really are.

In my opinion the information actually on the tire takes presidence. So unless you can get something in writing about your specific set of tires that states otherwise, the lowest speed you find on your tires is what your tires are rated for. For normal ST type tires this means 65 mph Max even if the number is not on your tires..  Not occasionally but ever.

I will publish an update when I learn more.


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Breaking News - Max Speed on ST type tires

Finally chased down the answer. Getting to the bottom was a challenge because I ran into "customer service" people who could read from a script but had no idea why they were giving me the answer they were.

Even got to an engineer at Tire & Rim Association and based on his answers it was obvious TRA had not thought about ST type tires or the unique nature of most RV trailer applications for many years so I believe he acted a little surprised when I identified the engineering analysis that indicated that the current loads too high for many trailers and that the load capacity should be decreased by 15% to 25% or more if the failure rate was to be decreased to a more tolerable rate rather than the 5 or 10% or higher some are reporting. This is not his or TRA fault as I understand it is the responsibility of tire company representatives to TRA to present and review technical matters.

Anyway here is the bottom line.

For Goodyear Marathon tires only:
There is a Goodyear  Tech bulletin   PSB#2011-13 that uses information from TRA.   
The tech bulletin says that the normal max speed for Marathon ST type tires is 65 mph. This speed can be increased up to 75mph only if the inflation pressure is increased by 10 psi. This new inflation can be 10 psi higher than the pressure molded on the sidewall of the tire that is associated with the tire max load.
The load does not increase beyond the load molded on the tire sidewall.
Goodyear does not support or condone operation above 75 at any time.
The trailer owner is responsible to confirm the rim is rated for the new inflation level.
While TRA indicates a further increase in speed might be achieved Goodyear has decided not to adopt this option.

The person I talked with at Tire Rack said they had already changed their web site to indicate this applied to Goodyear tires only and that they were attempting to learn if any other ST tire manufacturers were willing to make a similar change. As of Saturday Oct 19, 2013 they had not identified any other tire manufacturer willing to make an increase in max speed above 65.

++++++++++++++++++++++++++++
Info added 10/26/13 10:51 AM EDT
Example:
If you have a GY Marathon ST225/75R15 LR-D it is rated for 2540# @ 65psi and a max speed of 65 mph. If you increase the cold inflation to 75psi the tire would now be rated for 2540# @ 75psi and a max speed of 75 mph.
The example currently only is OK for Goodyear Marathon ST tires as I have not seen a published document from any other tire manufacturer. 

The pressure increase must not exceed the max pressure capability of the wheel. You need to find a max pressure rating stamped on the wheel or get something in writing from the wheel MANUFACTURER.
+++++++++++++++++++++++++++

My observation
This Max speed is like your engine red-line. While it might be possible to exceed this speed for short periods it will significantly reduce long tern durability.

I have sent a copy of this post to both TRA and Goodyear. If they ask for corrections or additions I will edit this post accordingly.