How much air pressure do my tires really need?

“How much air pressure do my tires really need?” Ya, I get this question a few times a month either here on RVtravel.com, or on my blog RVTireSafety.net, or on my Facebook page “RV tire News and Information”.

I have decided to not add to the confusion but to use this post as an example of how NOT to get the correct info on tire loading and inflation. Many times on the internet I find posts or threads or even YouTube videos where sometimes it seems that when someone learns a little bit about tires they think they know enough to publish an answer. It might have some of the words right, but most of the time there are some important facts or details missing from their “Words of Wisdom.” This does not mean that I think I know it all, as every few weeks I find myself reviewing information from industry publications such as these “Standards” as published in Japan, the U.S., or Europe.

They run from 400 to 550 pages each, with information on what wording is to be on a tire sidewall, as seen here.

Also, they include details on the dimensions of tire valves, as seen here.

They may even include something as relatively insignificant as the pin height of a valve core.

I used the last bit of information to resolve a problem with a TPMS not working properly for an RV owner.

Required information to get accurate answers on tire inflation

If you have a question on inflation, the following information is REQUIRED or the answers are just guesses.

1. Complete tire designation. This includes all the letters before and after the “size” numbers. The letter before tells us the “type” tire and normal application (P, LT, ST). Numbers and letters after the “size” numbers (C, D, E, G, etc., tell us the Load Range (Ply Rating), and the 3 digit numbers such as 123/135 give us the Load Index.
2. I don’t care about the “Speed Symbol,” as that only tells you how that tire, when brand-new, performed for 10 minutes on a test developed for passenger car tires with only 88% load. In my opinion, no tire in RV application should ever be run faster than 75 mph, with 65 mph being a much better and safer max operating speed.
3. Actual scale measured load on your tires. Truck scales only give axle loads, and almost no RVs have a 50/50 load split on the axle ends. Most are closer to 52%/48%, but some RVs have been found with over 1,500 lbs. more load on one end than the other. The only way to get the actual tire loading is with individual tire readings on a scale as offered by RVSEF or at some Escapees locations. Individual tire position weights aren’t as easy to get as the axle reading. So until you get the individual weights, I suggest you assume one end of each axle of the RV is supporting 52% or 53% of the axle load. At a minimum, run the inflation as shown on your vehicle Certification label.
4. You need to remember that the load capacities shown in tables and charts are the MINIMUM COLD INFLATION required to support the stated load. There is NO “SAFETY FACTOR” or “SAFETY MARGIN” built into the tables. Tire engineers that have or work with RV owners will many times suggest you run at least +10% margin on inflation by increasing the cold inflation. Or you can run at the limit of your wheels and decrease the loading until you achieve the 10% margin of load capacity greater than the measured load.

You might consider asking the person offering advice how many years they worked designing tires and what tires they have personally tested. Also you might ask if they have done enough forensic investigation and analysis to be considered an “expert” by a judge.

OR … you can do as some people do and believe that everything you read on the internet is true.

Good luck and happy camping.

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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