What size tire is it?

That seems to be an early response whenever you ask a question about tires. Whether you ask about price, load capacity, or inflation, the first response may be “What size is it?”  The answer you give should not be “It’s a 22.5”  or  “it’s a 225-R-15”. These are just partial answers and indicate to many that you don’t really understand much about your tires.

 The reason you need to provide the complete tire size information is simply because there are so many possible replies and starting to narrow down the possibilities by properly identifying the size is just the first step in learning the details so the person that is offering the help can provide the correct answer you seek and not just a wild guess. Knowing your tire size can be confusing and sometimes it seems as though we tire engineers and government agencies have conspired to make things difficult. What is needed is to remember there are three basic features that must be established first. Tire type, tire physical dimensions, and tire strength.

This article covers a number of different type tires. Please do not skip over any part as the knowledge provided here builds on previously covered information.

Let’s start off with TYPE. This is usually a function of the application. For most tires, there is a letter code as the first part of a “Complete” identification of a tire. For most tires sold in the US, the code for tire type is either a “P” for Passenger, “LT” for Light Truck, “ST” for Special Trailer, or no letter for commercial or heavy duty. Tire engineers sometimes call these commercial sizes “TBR”, short for Truck Bus Radial. If you are reading this article, most likely you have a Recreational Vehicle or RV of some sort so the use and application of tires on the various type of RV will be our focus. There are of course many other type tires. OTR for Off the Road or AG for agricultural or AT for All-Terrain or M for Motorcycle and others, but we will not cover those and will focus on the type, size and strength tires used in various RV application.

First, we will cover “P” type tires. Most of us own or have owned some form of a passenger car that came with P-type tires. Older and smaller trailers may also come with "P" type tires mounted by the manufacturer. When our car required replacement tires, we seldom needed to think much about the proper size nomenclature, as it was the responsibility of the tire dealer to confirm the appropriate type and size tire that was needed.  Our car tires would probably be identified as a P195/75R15 94S or similar combination of letters and numbers. The P, as you now know indicates Passenger car application. 195 is the width in mm.  Not the tread width but the maximum width. 75 is a ratio of the tire width to the tire height from the wheel to the tread. “R” stands for radial. Since there are very few non-radial “D” or “Diagonal” construction tires, we don’t need to go down the road of old tire construction. The "15" is the wheel size. Finally, there is the “Service Description” which is a combination of “Load Index” number, 94 in our example and finally, the “S” is the Speed Symbol. In the US, the speed symbol is really just an indication of the level of handling capability or steering response with increased handling potential as we move from Q to R, then S followed by T, U, H, V, W, Y and finally Z. Unlike Europe where by law you are required to replace tires with the same speed symbol, we have the option of changing the rating but should expect the steering response to get slower if we go to a lower symbol. The final bit of information concerning the use of P-type tires in RV application, the load capacity of a P-type tire must be reduced by dividing by 1.10 per tire industry design standards.

Next, we will cover "ST" or Special Trailer tires. This is a special type tire, unique to the US market. It was developed and introduced in the late ’60s for exclusive use on trailers. In fact, it is against safety regulations to use ST type tires on vehicles designed to carry passengers. In this category, we might find an ST205/75R15 101K LR-C  For these tires the ST205/75R15 101K is similar in meaning to what we saw in the Passenger type tires. The primary difference is that the Speed symbol K, L, or R is lower for these applications than for passenger type vehicle. The K stands for up to 68 mph. Because the ST type tires are expected to carry higher loads at higher inflation levels the trade-off is the restriction to be operated at lower speeds. The load formula used by tires engineers when designing ST type tires is based on a stated upper operating speed of 65 mph. While today highway speeds can be significantly higher than 65 we should remember that when St tires were introduced we had a nationwide Speed Limit of 55 mph so a tire design limit of 65 mph was not unreasonable. You should be aware that in 2002 both P and LT type tires had the test and durability requirements significantly updated and improved but ST tires only need to pass the same tests as in 1970.  In ST type tires, the abbreviation of LR for Load Range is introduced. You can think of the Load Range as a replacement for Ply Rating. This is really an indication of the strength of a tire to hold the inflation pressure, not the ability of a tire to support additional load It is important to remember it is the inflation pressure and not the tire construction that supports the load. This is why we have Load and Inflation tables not Load and tire construction tables.

 For RV application you will see that Load Range starts at C (old 6 ply rating) and moves through D, E, F, and G. Basically the Load range identifies the highest level of cold inflation to be used starting at 50 psi and moving up to 100 psi or even higher as identified on the sidewall of your tires. Unlike Speed Symbol you should never consider moving to a lower Load Range than selected by the RV manufacturer. The Load Range along with the original size, inflation level, and type tire is shown on the RV Certification Label that the RV company applied to your RV.

The next type tire is “LT” or Light Truck. These will be found on both Class-B and Class-C and possibly a few small Class-A RVs. Since these RVs are larger and heavier, they will normally come in larger physical size and stronger Load Range. For example, a popular tire for Class-C motorhome might be an LT225/75R16  115/112 LR-E.  The double number 115/112 is the different Load Index for single (front) application and dual (rear) application where two tires are mounted side by side. The Load Range (ply rating) has the same meaning as it does for ST type tires. As with the ST type tires, you should not consider moving to a lower Load Range than selected by the RV manufacturer. The LR along with the original size and type tire and minimum inflation is shown on the RV Certification Label.

Finally, we move to TBR tires as seen on most Class-A RVs. Generally, these are considered Commercial type tires and not consumer level tires. If you have and need this type of tires it is expected that you have a deeper level of knowledge about tires. Most of these tires come on 19.5 or 22.5 size wheels. They do not have a letter preceding the size description and may be something like 255/70R22.5  139/134 LR-G

These tires seldom come with a Speed Symbol but if you review the Data Books from the major companies that make most of the TBR tires you will see that they specify 75 mph as the maximum operating speed level. The double Load Index numbers 139/134 relate to the single and dual application and the Load Range letters continue to identify the normal upper level of cold inflation pressure.

There is a lot more information on tires available in tire Data Books and Industry Standards organizations. You do need to be careful about the source of the information you are relying on. Some sources, as in tire industry publications, can normally be relied upon as accurate but even there the information may not be aimed at the specific and sometimes unique needs of the RV community. There are many forums on the internet with hundreds of self-appointed “experts”. We need to be careful as just having used or sold tires for decades does not mean that all the information from that person can be relied upon all the time.

If you have questions about tires for your RV, it may be best to review some of the other posts on this blog.

Are you smarter than your tire salesperson? With Michelin LTX you may need to be.

As an RV owner, I believe that in most cases it is important for us to know as much as possible about our tires. Sometimes this means knowing more about our tires than the average salesperson.

Here is a real-life example:

An owner of an older Travel Trailer posted
"Please correct me if I'm wrong... The Michelin Defender LTX M/S 235/75R15 is a true LT tire and does not need to be derated?

Shopping for tires on my Airstream 2017 23FB. I'd love to just go with the 16" SenDel S02/Michelin combo but not sure if the clearance is there for them."

Just before the above post it had been pointed out  the correct designation is P235/75R15 XL
The "XL" is the tip-off as that stands for "Extra Load" which is only found in Passenger type tires.

LT tires have Load Range C, D, and E
P-type have Standard Load (no special marking)  and XL which is lower in inflation than an LT-C.

Your tire dealer should have made clear the type tire they are talking about.  Sometimes RV owners need to know more than the salesman if you want to get the tire you need or want.

I had also previously posted on that thread:
"Be sure you understand if your LTX tires are "LT" type or "P" type. If P you need to calculate the actual tire capacity when used on Trailer, SUV or PU-truck.

The adjustment is 
(Load molded on tire)/1.10 = Load capacity on the RV or truck or SUV application.

No load capacity adjustment is needed for LT type tires if placed on an RV, truck or SUV.The LT in "LTX" does not make a tire an actual "LT type tire."

I don't know if the marketing folks at Michelin realized the confusion they were spreading when they came up with the name "LTX" and put that designation on BOTH LT type and P-Type tires. I would not be surprised if there aren't a good number of RV owners who think they have an LT tire when what they were sold was actually a Passenger type tire.

Can I change from ST to LT tires on my Trailer or 5ver? Part Two

In Part 1 we left off with having to do some calculation.

I will assume you have confirmed the actual individual tire loading and have moved some heavy items around to end up with a reasonable balance of loads,

I will also assume you still want to change from ST type to LT type tires which means you must increase the Load Range and or increase the tire size to get a load capacity in the LT to match or exceed the capacity of the OE ST type tires.

Now before we move on you need to realize that "LT" is a designation used here in the US and European and some Asian countries have what they call "Commercial" tires. These Commercial sizes do not start with LT or CO but will probably look like 7.00R15  or for metric sizes 205/75R16C  The "C" in this case is not the Load Range but stands for "Commercial". The Load Range will be identified as normal LR-C, LR-D etc or possibly with RL or XL for Reinforced or Extra Load. To make this post easier to read I will limit my comments to LT type tires. Just remember there are other options that may be better for those with 15" or 14" wheels that do not want to change rims.

NOTE: All of these letters and numbers are important when selecting a size so be sure you record them all when doing your research.

So on to step
#6 Dimensions. There are two key dimensions Outside Diameter or OD and Width. Now I am confident that we all understand OD but width can be a bit confusing. Depending on the wheel well contour the overall maximum width or "Section Width may be most important. Some tires may have a narrower clearance nearer the tread so they will need to do some actual measurements at a number of locations.
It may be easiest to use the dimensions for OD and Section published for your current tires and just do a confirmation with your tape measure. Remember tire "width" is not the same as tread width.

You need to be sure the tires NEVER contact any portion of the RV frame wheel well or bodywork. You should try to have equal or greater clearance with the new tires that you have on your original size.

7. The challenge
When moving from ST type to LT type you will need to move up in Load Range or up in Size or both.

Now comes the research to see what your options are
8. Knowing the target Load Capacity and the maximum OD and Section width, it's time to use the Internet to do some research. The objective is to find tires that meet your needs for the numbers and that are appropriate tread pattern. You certainly don't need Snow Tires or heavy traction tread pattern. I would suggest that the tread be identified for "All Position" or Steer for your trailer application.
You can go to web sites from large dealers such as Tire Rack, Pep Boys, WalMart, NTB, Discount Tire or similar. You might also just Google "Trailer Tire" + the name of a large city or town near your location.
Once on their web site find the various possible tires that meet your needs.

9. If you are increasing the Load Range with the associated increase in inflation you need to confirm the wheel can manage that higher inflation. The info may be marked on the back side of the wheel or you may need to contact the wheel seller or manufacturer or you may need to get different wheels if your OE seller doesn't know what the rating is for the OE wheels

10 Finally, as I have previously suggested in my post on "The Best Trailer Tire" you need to make your purchase decision not just on lowest price but need to consider the tire warranty, even if there is a Road Hazard Warranty. Also how easy will it be to get a replacement if your tire gets a sidewall cut or unrepairable puncture.

11. Last step:  After all this work we want to do a first class job. Some might want to say you can't change from ST type to LT type due to Federal Regulations. Well a friend Dave Gray has an excellent post just on this topic. He has even provided an example of what would be the appropriate label you should apply after making modifications to your RV.


I hope these two posts have helped you understand the steps, calculations, measurements and research needed to make such a change.



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Can I change from ST to LT tires on my Trailer or 5ver?

This question is a real "Hot Button" on a number of RV forums and blogs. People ask this question because they want a better or alternate selection of brands or they want to improve the durability of their tires.

The answers given seem to range from "Sure, Why not" to "Absolutely Not, Never do it" and some will even offer that they think you are breaking some law if you make any change from the type, size, Load Range or cold inflation from the OE tire information shown on the tire Placard.

As we all should know by now answers about tires are never simple and straight forward and changing tires is definitely one of the more involved answers.

First off, I am not a lawyer but an Engineer and as such I form opinions based on data and facts. So here is the answer based on my 40 years experience as a Tire Design Engineer.

Yes you may be able to change tires  

    

 there are some things you MUST do to ensure that any change you make will actually improve your probability of having better tire durability.

My plan is to provide an outline of the steps you need to take before you make any change. I will try and include each step and each of the points of data you need to collect and evaluate. If you skip a step you may end up with a less durable tire selection which could lead to tire failure, RV damage and even an accident.

Before we start you need to consider that the most conservative approach is to make no change and to simply use the tire Construction (Bias or Radial), Type (ST or LT) and size and Load Range and inflation as specified on the placard and specification documents. This represents the RV manufacturers recommendation based on a number of assumptions as well as some legal regulations the RV MFG must follow plus in many or some cases a desire on the Mfg part to keep their costs as low as possible.

So if you still want to move forward here are the steps you need to take:
1. You need to know the actual load on each tire. This is important because A. we will be basing some decisions on the tire loading    and     B. It is possible that there is a significant unbalance in the tire loading which may be the cause of poor tire durability. With sufficient unbalance it may be impossible to provide a tire selection that would lower the probability of having problems.
To learn the actual individual tire loads you need to either find a company such as RVSEF or agency that has individual scales or to follow the steps outlined on worksheets such as This one  or This one. I have hears some people say that they have been able to get the individual tire loads from their state police or state DOT.

2. Knowing the ACTUAL LOAD on each tire you need to confirm that no single tire is loaded more than the max load molded on the tire sidewall. This is an absolute rule. If any tire is overloaded you should not move the trailer until you either change the load or change the tire.

3. Assuming no tire is overloaded we want to make sure that all tires on en axle are inflated to the same inflation. For multi-axle trailers you can lower the internal tire structural shear forces (the forces trying to tear the tire apart) by running the inflation molded on the tire sidewall. Sometimes this is stated as the Max PSI and other times it is stated as the PSI for the max load. For our purposes we will consider this the proper cold inflation you should always run. 

4. We should have "Headroom" or "Reserve Load" or "Safety Margin" on the tire loading. I suggest 15%. However I know that for many trailers 15% extra capacity above the actual load is very hard to do. Especially since some RV MFG manage to so under-size the tires that even when the trailer is empty they may not have 15% margin.

5. If you don't have at least a 10% margin I would strongly suggest you need to consider changing tires to something with more load capacity when inflated tot he sidewall PSI.

OK so you have some homework to do. We will continue this in the next post.


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Why did my engine "blowout"?

Why is it we almost never hear the above question on various RV forums?

Could it be that we don't hear this question because people know it is not wise to exceed the RPM limit or "Red-Line" specification of their engine so they simply don't run at and above the engine speed limit?

But I do see questions of what has to be done to run 75 mph with a standard ST type tire when the RV manufacture specifies the pressure on the tire which is one of the factors limiting the tire speed capability.


The interaction of tire types, load capacity, inflation and speed "rating" are complex, as are almost all topics about the various performance trade-offs with tire selection

I did a post just on the topic of "How fast is safe to drive on your tires?"   I have also pointed out that if you are going to deviate from US industry guidelines, such as published by Tire & Rim Association, you must follow the published guidelines for your specific size, type and brand. You cannot use Goodyear document to learn the appropriate load & inflation for your Maxis tires. It even means you cannot apply specifications, including speed, load & inflation from a Goodyear Marathon with a Goodyear G614.

Now as we all know, an ST type tire has a higher load capacity rating than a similar sized LT type tire.
ST235/75R15 LR-C is rated 2340# @ 50 psi (min) and 65 mph (max).
LT235/75R15 101/104Q LR-C is rated 1985# @ 50 psi (min) and 75 mph (max).

We have also established that load capacity is nominally a function of tire size (air volume) and its inflation pressure. If however we decrease the tread depth and the lower the maximum operating speed we can increase load capacity slightly. In my post of Oct 12, 2011 I identified a Michelin trailer tire with a higher load capacity but many want to ignore its max speed rating of 62 mph which is even lower than ST type tire rating of 65 mph.

Now to the specific question of how to operate a specific tire at 66 to 75 mph. This is accomplished by increasing the inflation pressure by 10psi according to a document published by the tire manufacturer. This is not as simple as many assume.

If you select the 65 mph max speed for an ST245/75R16 LR-C you will find a load capacity of 2,600# with a minimum of 50psi if you stick to 65 mph max. But if you want to drive at speeds up to 75 you need to increase the inflation pressure in the industry standards as published by Tire & Rim Association. So now to carry 2,600# you need an inflation of 60psi but this exceeds the max inflation rating for this size @ LR-C so you either need to get new LR-D so you can run 60psi or to limit your load to 2,270# and 60psi which is the published load capacity for 50 psi inflation of the subject size tire. This higher inflation may also exceed the rating of the wheel, but again many choose to simply ignore that rating too.

One obvious item many simply ignore is that the max speed for a tire is much like the red-line rev limit for their engine. It clearly is possible to run an engine with a 6,000 red-line at 6,300 RPM and possibly higher but what about long term engine life?

There are hundreds if not thousands of posts on RV forums from people complaining about tire failure but I have to wonder why they seem to understand the effects on engine life of high RPM operation but fail to understand that running a tire faster than its design specification will also contribute to shorter tire life.

What does “XL” mean?

In this series I am giving a lot of detailed technical stuff. For some of you this will make your eyes glaze over with boredom while others really want to know the “Why” of different tire selection. Please bear with me as I try and build a foundation of information that anyone can come back to later if they develop an interest or have a question.

Last time we discussed “P-Metric” sizes. That means tires whose size begins with the letter “P” and have metric width dimensions. P235/75R15 the 235 is millimeters and the size starts with a P so that is a “P-metric” size.

In the P-metric post we discussed a P235/75R16 105S which was rated at 2,028 Lbs at 35 psi max. Before we move to Light Truck tires where things get a bit more complex with multiple Load Ranges we need to finish up our discussion of Passenger tires. These are “Standard Load” tires with a max inflation of 35psi and “Extra Load” with a max inflation of 41 psi.

Since most passenger tires are standard load, that is the default so few tires actually have the words “Standard Load” on the sidewall. However if the tire is rated for Extra Load it will have the words “Extra Load” and possibly “XL” on the sidewall in addition to the 41 psi max information. Originally Extra Load passenger tires were used on Station Wagons that some of us might even remember. In the 60’s there were regular passenger cars and Station Wagons.

SUV’s, Crossover or other type vehicles really were not part of the market.

Since these larger vehicles were intended to be used with higher loads most of the time, it was soon discovered that there needed to be an adjustment when a tire was selected. Industry guidelines indicate that the vehicle normal load on the tire shall not be greater than 88% of the max load capability of the tire.

Additionally if a P-metric tire is used on a “multi-purpose” passenger vehicle, think SUV etc or truck, bus or trailer, the load rating shall be reduced by dividing by 1.10. The total load capability at the recommended inflation shall not be less than the Gross Axle Weight Rating or GAWR.

I guess you can see that things get a bit involved for the engineers at car manufacturers when they try and select an appropriate tire size and inflation.

Extra Load rated tires are one option used by the engineers when selecting tire size. In our example size P235/75R15 108S XL we can carry an additional 308 Lbs on each axle at max inflation.

If your tire placard or owner’s manual indicate your vehicle should use P-metric XL tires then that is what you should use.

I’ve got a short list of reader questions to answer, then we can move on to other type tires that many will find on their Pick-Up, Trailer or RV.

What do all those letters mean in the tire size?

A fairly consistent problem I try to provide answers about specific tire applications, is the fact that we are usually only given part of the tire information we need to provide an informed answer. I have been asked “What load can my 16” tires carry” or “I have 8 ply tires, how much load can they carry?” or “Can I replace my 235/70R16 tires and carry more load with a 245 tire?”
While I really want to help you get the information and answer you seek, I do not want to guess what tire you have as I may guess wrong and give you incorrect information.

The reality is that the numbers alone are not all the information we need. To help us do a better job of answering your questions, you can help us by giving us the needed details.

Here are some examples of how you can help us all if you provide complete size information when you ask a question.

A P235/75R15 105S is rated at 2,028 Lbs at 35 psi maximum. This would be a “Standard Load” tire, while a P235/75R15 108S XL or Extra Load is rated at 2,183 Lbs at 41 psi max.

We can break down the different parts of the complete size designation as follows. The P stands for Passenger. The 235 is the width of the tire in millimeters and the 75 is the Aspect Ratio of how tall a tire is relative to how wide. In practical terms it is how close the wheel is to the road. The “R” of course stands for Radial and if the tire were Bias as tires were before the introduction of Radials it would have a D for Diagonal. Don’t ask me why they didn’t choose the letter B. I have no idea.

We all know the 15 is the rim diameter. The 105 or 108 is the Load Index. You can consider any passenger tire to be Standard Load unless marked “XL” or “Extra Load”. The “S” in the size above is the speed or handling rating. Passenger tires are not rated for dual application. The combination of Load Index and the Speed rating make up the Service Description.

When you ask a question about a tire don't worry about what all the letters and numbers mean just look at your tire and copy all of the information and leave it up to me and other tire engineers to decipher the mumbo-jumbo. If you tell us your tire size is a P285/35ZR19 87Y we will know this ultra wide, very low aspect ratio large rim tire with a high speed rating indicates you are driving a Corvette and that those tires are not meant to be used on your RV.

Next time we can move on to other type tires that many will find on their Pick-Up, Trailer or RV.

Light Truck tires on trailer

Had a question from a reader
Tireman:
Thanks for your expertise. I am, frankly, baffled about what to do about our tire situation. In this instance, our tire that went flat definitely was not overinflated. It had been checked about two days prior and had 63 pounds. We have been running nitrogen since the tires were put on and have not lost a pound in any of them in 3 months.
When we upgraded to 80 pound, E-rated tires three years ago, I asked about the rims' ability to hold that pressure. Stupidly, I took the tire dealer's word for it.

In 2008, we started blowing Denman tires all over Montana: three of four blew, tires with about 8,000 miles on them.
On our way to Maine in May, we blew yet another tire and sought advice from the guy who owns the campground where we were headed. He is a tire dealer, and advised us to go back to the original equipment 65-pounders, which we did. Now, we have a rim problem.
An 80-pound Maxxis, which we kept as a spare, is now on the trailer. The one that went flat will be on a new rim and used as a spare until we get home. Clearly, we had lost pressure in that tire before we got to the CG; how much is the question. It was not flat or we would have noticed it when we put the chock between the wheels. That tire will undoubtedly go bye-bye when we get home.

I will research rims. It sounds like I should replace all rims with ones that are capable of handling E-rated tires if we want to go that route again. I do not think we have room to upgrade to 16-inch wheels, but I now am going to find out for sure. If it's possible, we will do that.

I am adamant about checking tire pressure, so I am confident we have not been running on over or under inflated tires. We are not overweight, and our axles are fine.

It is obvious we are doing something wrong. I'm beginning to feel like we are too stupid to own this 5er!

Hope your eyes don't glaze over reading all this...thanks for your input.

Answer
You didn't mention if you ran 65psi or 80 psi in your "80 pound" tires. I wonder if you are running metal valves, as standard rubber valves are not rated for more than 65 psi. Also the rims have a max load and max inflation rating. Did you check the rim stamping?

You also were not clear if you were running LT or Light Truck tires or had been running ST or Special Trailer tires. These two different type tires have different load ratings even if the rest of the numbers were the same. A LT235/75R16 Load Range E has a different rating than an ST235/75R16 Load Range E.

An "80 pound" Load Range E tire rated for 80psi will carry no more than a Load Range D tire will if they are both inflated to 65psi. It is the air (nitrogen) that does the work and carries the load not the tire. Just because you checked the tire two days earlier is no guaranty you didn't get a nail as you left the CG that day and drove for two days on a leaking tire. This is one of the best arguments in favor of TPMS.

With all the tire failures I have to ask what your real ( not sticker or guess) axle by axle side to side loads are when you are fully loaded going down the road. With those numbers you can consult the manufacturer’s charts.

While you are getting the TT weighed it won't hurt to get the four corner weights of your tow vehicle too.

One thing few realize, including the tire salesman, is that according to Industry Guidelines LT tires require an inflation increase or even a load reduction if you ever drive over 65mph. ST tires also have a speed restriction that needs to be considered. The only way to know if this is not needed is to consult printed literature for your brand and design tire from the tire manufacturer. I would not assume the tire salesperson knows the correct answer.

Load Range - Ply Rating - Standard Load and related markings

Had a good question from George. He said "The tires on my Chevy Silverado say "Standard Load" rather than Load Range. What does Standard Load actually mean?"

The simple answer is that your tires are rated equivalent to Load Range B. But I am sure that raises more questions.

This is a good time to clarify some of these terms. To do that we need a bit of a history lesson so we understand why the term "Ply Rating" is out of date.

80 years ago when tires had cotton for the body fabric four Bias plies were needed for use on passenger cars. Trucks needed more air to carry their heavy load which meant more layers or plies were needed to hold the increased air pressure, so truck tires had 6, 8, 10, or more actual layers of cotton body fabric. They needed the even number because they were Bias not Radial construction.

With the invention of synthetic materials such as Nylon, Rayon, Polyester fewer layers or "Plies" of fabric were needed to hold the same air pressure. The term "Ply Rating" came into use when the actual number of plies of these synthetic materials was less than the number of cotton plies.

I remember when I started working in truck tire design in 1969 using terms like "6 for 8" meaning we were using 6 plies of Nylon to deliver the strength of 8 plies.

With the introduction of Radial construction from Europe a new size standard based on actual tire dimensions was used but something was needed to address load so about this time the use of "Ply rating" was replaced with "Load Range" and a letter was used instead of a number as the number was misleading the consumer. We ended up with Load Range, or "LR" B replacing the number 4 and C replacing 6, D replace 8 etc. Since essentially all passenger cars came with Load range B tires it was decided this was the "Standard" load and this did not need to be marked on the tire. There are a few "Extra Load" passenger type tires which you could consider like a LR-C and they are marked as such.

All Light Truck and Heavy Truck tires are marked "Load Range x" with the x being the Load Range for that tire. They may also simply have the Load Range letter right after the size.

To further confuse things there is now a "Service Description" which includes the speed rating. So you may have a passenger tire marked:
P205/75R15 84H The 84 is the "Load Index" and the H is the Speed Symbol.

A Light Truck tire might be LT205/75R15 98/95Q LRC with 98 being the single Load Index and 98 the Dual Load Index and Q the Speed Symbol and LRC indicating the tire is rated a Load Range C tire.

A heavy truck or bus tire might say 255/70R22.5 LR-G 138/134M At this point I trust you can figure out what the letters and numbers mean. If not and want more info check out
this link or simply ask the question.

Tire design goals: what are they?

Usually a sales and marketing department decides they want a new tire design or size based on expected needs or wants of customers. The customer can be the end retail buyer or in some cases a vehicle manufacturer.

If it is a vehicle manufacturer such as Chevrolet, Ford, Toyota or any other car or light truck manufacturer, they will have a long list of performance characteristics they want for their new vehicle. Some of these will include measurables such as weight; high speed capability; wet, dry and snow traction; noise characteristics on different road surfaces; ride characteristics also on different road surfaces; rolling resistance (fuel economy) and of course pricing, quantity and delivery volume rates.

Since most Class C motorhomes are built on what is called a “cut-away” van chassis, the list will have many but not all of the same type of performance measurements, albeit with different numbers.

It is important to remember that when a van chassis is designed the manufacturer does not know if the vehicle will be used as the base for an RV, ambulance or delivery truck. So the tires are normally not specified to provide performance specific to a certain type of vehicle, but a compromise for use on all of them. Class-B motorhomes basically use regular delivery vans which are modified with RV features.

If the vehicle manufacturer makes a heavy truck chassis like those under Class A motorhomes, their list is much shorter and may only include a couple of characteristics. In some cases they will select a tire from existing production so the only details to be worked out are pricing, quantity and volume delivery. Travel trailer manufacturers usually select tires that meet the expected load capabilities of the trailer and purchase in bulk from existing inventory.

NEXT TIME we will cover the process of taking a tire design from concept and “wish-list” as we turn it into a prototype. Later we will cover manufacturing then we can move on to performance, tire care and maintenance and finally tire problems.