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Monday, June 26, 2023

What does "MAX Inflation" on tire sidewall mean?

 "MAX PSI" is a hot topic on various RV forums I monitor, so let's jump in. I have answered this question previously but some folks have decided they know more than the tire companies.

First, as always, you need to remember we are 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 two 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 one happens to be from the side of an RV trailer. 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.

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 LT 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 car 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. 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 is 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.

Roger Marble

Check out my Blog www.RVTireSafety.Net

Have a tire question? Ask Roger on his RV Tires Forum here. It's hosted by and moderated by Roger. He'll be happy to help you.

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