Does your tire says "MAX PSI 80 psi." or something similar?

 My answer is that while those words or similar may be on the sidewall, those words are NOT to be taken literally as an instruction because those words are in reality only PART of an important phrase.

In reality your tires more likely say something like "Max Load 2500 pounds at 80 PSI Max Inflation". Of course different vehicles will have different size and type tires but we are looking at some general guidelines here.

Do you see and understand that the actual information the tire company is telling you is that the tire has a maximum load capacity of 2,500 pounds when it is inflated to 80 Psi? Most people understand that the load capacity of a tire is controlled and limited by the inflation in the tire, so this phrase is telling us that the load capacity of the subject tire will never be greater than 2,500 pounds even if you increase the inflation pressure

The loads and inflation are published in industry standards books such as the US Tire & Rim Association. Almost all tires made for sale and use in the US will follow the industry standards book AKA "TRA".

 I believe that the RV company engineers either have copies of the books or at least copies of the applicable pages.

 As always, you need to remember we are ALWAYS talking about the "cold" inflation which means when the tire is at Ambient air temperature and has not been driven or in direct sunlight for the previous couple of hours. If I ever talk about the warm or hot inflation that will be made obvious in that post.

Every vehicle has a Certification Label. This image is from a Class-A RV.

The specifics of this individual label are not important as you should have captured a nice, sharp picture of the label for your RV.  Class-A has the label usually near the driver's left elbow, Class B & C and LT have the label in the driver door jam and towables (TT and 5th wheel trailers) have the label on the outside, driver side, toward the front of the trailer.

The label states the Tire Size, and the recommended inflation if it is a passenger car. If it is an RV it will give the GAWR, Tire Size, Tire Load Range, and the inflation required to support the GAWR. Passenger car and pick-up truck inflation is arrived at after years of testing and evaluation of different tire designs from different manufacturers at different inflation that balance the requirements on a long list (hundreds of items) from the vehicle company. These requirements include Force & Moment test results and fuel economy plus dozens of detailed ratings within each category for Ride, Handling, and Noise. Car inflation is a "Recommendation" from the engineers at the car company aimed at giving you the performance they designed into the car.

I have never heard of RV companies evaluating tires. It seems that they generally look for the lowest-cost tire that can provide the load capacity required by law. This usually means for towables that the tires would need to be inflated to the level required to deliver the max load capacity. A few RV companies are selecting better tires with Nylon Cap Ply but you need to pay attention.

The inflation number on a tire sidewall is the PSI needed to deliver the Max Load rating stated on the tire. Normally tire load capacity is increased with an increase in inflation and that would normally mean you would increase the inflation, BUT since the tire was selected for its max load capacity as stated on the tire sidewall, the inflation that will deliver the highest load capacity for that tire is the "MAX infl" which is NOT the highest inflation the tire can tolerate.

Yes, the wording can be confusing but the wording was not selected with the average user in mind.
Motorhomes are built on chassis built and designed by vehicle manufacturers so the tires on Motorhomes will many times have inflation on the label that is lower than the tire max because the tires were evaluated and selected by the vehicle chassis manufacturer.

Many towables are built with minimal capacity for additional "stuff" but many people load their RV with no thought to the load capacity. This is why the data shows that over half the RV on the road have one or more tire or axle in overload. This is a major contributor to tire failures and why there are so many RV trailer tire failures.

Too often on RV forums, people do not pay attention to what type of vehicle is being discussed A Passenger car (or 1/2 tom Pickup) or a Motorhome or a Trailer (5th wheel). These three completely different types of vehicles require three different guidelines and inflation information. This failure to be specific with the vehicle type is why people get confused.

 

Do you run at or above engine Redline? Why Not?

 OK As a tire design engineer and someone with RV tire experience let's see if I can clear up some of the partially correct and partially misunderstood information we see posted almost daily on one or more of the Internet RV forums.

If you have a Load / Inflation chart for "ST" type tires you will find that ALL ST type tires follow the same information. A chart published by company X may not have all the same sizes and Load Range tires as company Q but when both X and Q make the same size and the same Load Range the load capacity and the required inflation for the tires is identical.

 

The same applies to "P" and "LT" type. Now we need to pay attention to some other details on tire "type". Some tires made outside the US are not marked with a P, LT or ST before the size numbers. This means that they were made to some different standards. They may have been designed to comply with European or Asian standards to their numbers will be close but not identical to the US standards.  

 

The inflation number in the chart is the MINIMUM inflation required to support the stated load or to put it another way the LOAD in the chart for your tire is the MAXIMUM load the tire can support when inflated to the specific inflation found in the chart.

 

Some larger sizes with 19.5 or 22.5 size wheels make things a bit more difficult as some follow the US Standards while others follow European standards but there is no leading letter to help you know which so if you run tires of these sizes it is more important that you learn the Load capacity for the different inflation for these tires directly from the company that made those tires. The differences are not great but they are not identical so you can be off by 5 to 10 psi or maybe a couple hundred pounds load.

 

RV companies are required to select tires that can support the GAWR but trailers have different "assumptions" than do Motorhomes. For trailers the US DOT assumes a perfect 50/50 split of the load between two axles or 33% each when there are three axles on a trailer and also a perfect 50/50 side to side load split on any one axle. Neither of these load splits are realistic as those of you that have had each tire positions measured know that many axles have end to end load splits of 48/52 to 40/60%. Individual axles on multi-axle trailers can also have significant variation away from a uniform load split.

 

Motorhomes also have similar problems with the unbalance of each end of each individual axle. Engine placement and dual tire position on the rear axle require paying attention to other details when consulting Load &Inflation tables as "Single" load capacity is always different than when tires are is a "Dual" or side by side mounting as seen on most motorhomes.

Size and placement of water, fuel, propane, and holding tanks can result is 500# to 1,000# being in different corners of different model or different year motorhome so that is why you can't simply ask the RV owner parked next to you about proper tire inflation numbers.

The inflation number on most Certification labels is the MINIMUM inflation needed to meet the above requirement.

A complicating issue is that in 2017 RVIA (Gold oval sticker on the side of most RV trailers) added a requirement that the tire inflation be sufficient to support 110% of the GAWR. This partially addressed then known in-balance found on almost all RVs.

All the above ignores the advantage of having some "Reserve Load" capacity to allow for variation in loading of the RV that can occur if you carry more water than normal or load more "stuff" than normal etc. It also ignores normal engineering practice of not designing components to have some level of margin of load capacity. or for the occasions when trailers are towed at speeds above the tire design limit of 65 mph operating speed.

 

You can think of running at the max load capacity like running your TV right at the engine Redline. If you think that is a good practice to run with no reserve load then I guess you also must think it OK to run at engine redline for hours on end.

##RVT1058

Your tire says "Max xx psi at Max Load of yyy Pounds" Will your tire blow up if you go above xx psi?

 The simple answer is No. Your undamaged tires are not going to Blowout or Explode or Blowup if you see a pressure greater than xx psi on your TPMS or on your hand gauge.

There is a lot of confusion out there because people do not understand the reason for the confusing wording that is mandated by DOT.

There are Federal regulations on the words and information that must be molded on the tire sidewall. This wording has been around for years with some unchanged since the 1960's.

A recent poll of RV owners responding to a question on tire inflation number on the tire sidewall indicates that 18% think the inflation number molded on a tire sidewall number is the absolute highest a tire should ever have in it. Another 18% think that inflation is "the best" inflation for the tire 2% think it's the lowest pressure the tire should ever have. I am very disappointed with this level of confusion.

Here is the reality:

Each type and size tire and Load Range has a stated Maximum load it should ever be subjected to. The number is molded on the tire sidewall in both pounds and Kg. The tire industry has published tables that provide the MINIMUM inflation a given tire needs to support a stated load. The tables clearly state that the inflation number is the inflation measured before the tire is driven or warmed by direct sunlight. This is called "Cold Inflation". Not "Refrigerated" inflation and not some laboratory 68F or 70F "standard, but the inflation that would be the same as the surrounding ambient air. Some people know this as the "Temperature in the shade".

The confusion comes about because until recently vehicle owners never knew the operating temperature and pressure of their tires. However with the introduction of aftermarket TPMS as used by many RV owners, they now have those numbers presented to them.

What is missing are two things. One being training by the selling dealer as to what inflation is needed to support the stated load and second an explanation of what the words on the tire actually mean.

I am not sure if the RV salesman has ever received the training other than to tell the customer the information is in the Owner's Manual.

Hopefully when an RV owner reads "Max Load"  they understand that they should never load the tire more than that.

The confusion comes with the inclusion of the word Max as it relates to tire pressure.

IMO the wording would be much better and more logical if the tire said "Max Load yyyy pounds at xx Psi cold".

I would leave it up to the people at DOT to try and explain why they were not consistent across all types of tires with the wording on load and inflation limits, but I have no idea who to ask. as I expect them to pass the question off and say "Ask the tire manufacturer" but the manufacturer is only following the regulations established by DOT.

I have a large number of posts in my blog that mention inflation. If you have questions I can suggest you review my posts as the questions are asked a few different ways and I provide the answer with what I believe is a consistent interpretation of the intent of the requirements.

 

##RVT1042

Should you use the inflation that is listed on the tire sidewall?

 Many RV owners do not understand the inflation and load information that is molded on the sidewall of the tires they own.  As a tire design engineer, part of my responsibility was to specify all the words, symbols and numbers on the tire sidewalls. Some of the words such as brand name like as Goodyear or Michelin and the "line"  like  "Eagle" or "Transforce" are obviously selected by the sales and marketing departments. 

The tire size or sizes had been selected and it was my job to design and request molds appropriate for the sizes that were to be part of the new line of tires. For each size, I needed to consult the US Tire & Rim Association data book and to specify the appropriate load and inflation numbers for the maximum allowable for the Load Range of each tire. With LT lines we might have LR-C, LR-D and LR-E tires and if incorrect numbers ended up on the tires they would need to be recalled and replaced for free so it was obvious that this information had to be correct.

With the exception of a few tires that are designed for a new model of a specific car or Light Truck from Ford, GM, Toyota, M-B etc, I would never know what vehicle the tires would end up being mounted on so there would be no way for me to specify a specific load or inflation for any given tire that was vehicle specific. Selecting the tire and Load Range and specifying the inflation number to be posted on the vehicle Certification label is, by regulation, the responsibility of the vehicle manufacturer. It was however my responsibility to ensure the tire was capable of passing the DOT tests as well as company minimums based on the use of industry standard loads and inflation numbers.

All tires have a Maximum Load capacity molded on the tire sidewall along with the minimum inflation needed to support that load.

With this background information I hope you now understand that the inflation number molded on the tire sidewall is not selected or made up by the tire engineer as the inflation number comes from the published Industry standards. The inflation on the Certification Label aka Tire Placard is specified by the RV manufacturer and by federal Regulation must be sufficient to support the GAWR (Gross Axle Weight Rating) which is part of the RV company design process.

The placard inflation may be equal or less than the number on the tire sidewall but it should never be higher than the tire sidewall pressure number.

Ideally you would have actual scale loads with the RV fully loaded at its expected heaviest. With that load known you can consult the tables to learn the minimum inflation needed to support your load. However if you don't have scale readings the best thing to do is inflate your tires to the placard information. This is OK as long as you have not overloaded your tires. Note that the tables for tires made by Goodyear are the same as for tires made by General, or Firestone, or Bridgestone, or Cooper. Also most Michelin tires have the same numbers as the other tire companies with the exception of a few tires that were originally designed for European country application where metric units are standard. When converting to the US "Inch-Pound" units the numbers do not always match up so if you have Michelin tires you probably need to consult their tables.  You can also check the tables listed on this blog post.

 

If you review the above you will see that I am not advising that you use the inflation on the tire sidewall, unless that is the inflation on your Placard.

A side comment on the inflation number of the tire. The words may be "Max Inflation" on some tires, but this is related to the way loads are established for tires based on the inflation. The LIMIT is the stated maximum load. The intent in the wording is to stress that the tire load can not be increased even if the inflation is increased above the number associated with the tire's "Maximum Load".

Regarding hot tire inflation numbers higher than the number on the tire sidewall, that is OK as long as the set "cold inflation" is not set higher than the number on the tire. Tire inflation will increase when the tire temperature increases. The rate  of change is about 2% for a change of 10F. Seeing your "Hot" inflation higher than the tire sidewall is not a big deal. With the advent of TPMS many people are seeing tire temperature or pressure increase for the first time as a driver. Most TPMS have the high temperature set to 158F.  I have found that setting the high pressure to 125% of your cold set pressure should normally avoid having the alarm sound as long you are not overloading your tire or pushing your driving speed. If you get close you might just drop the speed back a few mph. Personally I set my cruse to 62 or 63 mph and stay in the right lane. I do not get people honking and I get real good fuel economy. (10.2 mpg in my last 1,200 mile trip from Ohio to WY).

 

##RVT1008