OK, I must apologize for the corny title.* But as I monitor a number of RV forums on the internet and on Facebook I often see comments to the effect that, “This is just another post on inflation, so why bother reading it?” But there also seems to be a never-ending stream of new RV owners who do not know the basics of proper tire maintenance and that proper inflation is a critical part of that maintenance. [*“Danger, Will Robinson” is from the ’60s TV series “Lost in Space.”]
This is from owners of one brand of RV towables, but I see similar comments on forums from other brands, too. After seeing 39 posts with at least 30 different answers on how and when to set tire inflation, I decided to jump in and offered the following:
First off, you might care to know that I am a retired Tire Design and Forensic engineer with 50+ years of experience designing, testing, working with, and inspecting Truck, Passenger, Indy, LT, and ST tires.
Next, 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.
To answer the question of “MAX PSI” on a tire sidewall mentioned in a number of posts:
Every vehicle has a Certification Label. If it is a passenger car the label states the Tire Size and the recommended inflation. If it is an RV or truck it will give the GAWR, Tire Size, Tire Load Range, and the inflation required to support the GAWR. Passenger car inflations are 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 (100’s 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, Noise, and Vibration. Car inflation is a “Recommendation” from the engineers at the car company aimed at giving you the performance they designed into the car.
RV companies, as far as I know, do not evaluate tires as car companies do. They generally look for the lowest-cost and smallest 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.
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 the max load capacity stated on the tire sidewall, the inflation that will deliver the max load capacity is already on the tire.
Yes, the wording is confusing, but the wording was selected by the lawyers, not tire engineers.
Motorhomes are built on chassis built and designed by vehicle manufacturers, so the tires on motorhomes will many times have inflation lower than the tire max.
Many towables are built with a minimal capacity for additional “stuff,” but many owners load their RV with no thought to the load capacity. This is why the data shows that over half the RVs on the road have one or more tires or axles in overload. This is why there are so many tire failures.
Bottom Line
1. Check and set your pressure when the tires are “cold.” That means when they have not been driven on or in direct sunlight for the prior 2 hours.
2. Select either the pressure on your Certification Label or the pressure in the tables based on your scale weight. If you use the tables, add 5 psi to get your “set” pressure number.
3. Set the Low-Pressure warning level on your TPMS to no lower than 5 psi below the psi in step #2.
4. Never bleed down the hot pressure once you have established your minimum cold pressure.
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 RVtravel.com and moderated by Roger. He’ll be happy to help you.
Read more from Roger Marble on RVtravel.com.
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Excellent article. I have been advocating scale weights and Load/Inflation tables for years. And after 6 motor homes and a few tire issues along the way, I do consider myself more of an expert than most. But of course there are still lots of folks that tell me I’m wrong and that the only pressure to use is what is recommended by the motor home manufacturer. Sad. Obviously the manufacturer has no idea how much junk (or not) an RVer will load in their coach or trailer. My other strong recommendation is to get a Tire Pressure Monitoring System. We have had 2 different brands and they have saved ruining tires on a number of occasions. Keep up the good work.
The placard on the side of the RV is for the tires it was shipped with. If you have upgraded, say from a C rated to a D rated tire, wouldn’t you go by what is on the tire and ignore the placard? I have never seen this addressed.
As Roger said, the placard on the RV is for the tires it was shipped with, at the GAWR (maximum weight the axle is designed to carry.) If you are talking about going from load range C to load range D, you probably have a pretty small RV without a lot of Occupant and Cargo Carrying Capacity. In that case, you may be close to the GAWR and the wheel weights may require you to run the load range C tires at the sidewall pressure without the recommended 5 to 10 psi “reserve.” Going to load range D tires allows you to run a slightly higher pressure based on the wheel weights plus the “reserve” but you don’t have to go all the way to the new sidewall pressure.