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Friday, June 18, 2021

Are tire pressure recommendations changing?

A question from an Airstream owner.

Hello All,
I have a 2020 Ford F250 and an Airstream 33' Travel Trailer. I've read a lot of discussion about tire pressure, but nothing real current. What I mean is, the newer F250 door column states for these Michelin tires now, 60psi front and 65psi rear. The tires max at 80psi on the sidewall which was my experience with prior year F250's on the door column too, but the tech at the dealership said they've changed and that with normal driving you can run 80psi, but harsher ride and you'll run the "middles out of them."

On to the Airstream. All four tires are Michelin LT225/75R16, Load Range E. The Airstream sticker says 80psi for these tires. I went up to my local Dobbs Tire Dealer and he pulled up the Michelin chart for these tires, and said with a trailer weight on dual axles of between 7980lbs and 8600lbs, you could run the pressure between 60 -- 65psi. I asked if I would have a problem with tire heat or wear and he said no. I did put a 3" lift kit on the Airstream, so as to have better clearance at gas stations and various road conditions. The dealer puts in 80psi, but from time to time I get popped rivets, (if ride is the cause).

My question is this: With an approximately 8200lb trailer, 1300lb hitch weight, and just me and some luggage, (300lbs of luggage), what would you recommend on tire pressure on both vehicles for mostly interstate driving, despite the door stickers, (if recommended)?

My reply:
Not sure why you think the information on appropriate tire pressure is different today than it has been for the last 10 years.

TV Car companies have teams of engineers that work closely with the team of tire engineers to select tires and inflation numbers that will deliver the best overall tire and vehicle performance. The consider and test for ride, handling, steering response, fuel economy and safety in emergency situations. The inflation on the sticker along with tire inflation information in the owner's manual should be followed. Mechanics and service people at the car dealership are not involved with the selection or testing and evaluation of the new tires that were selected for the new vehicle. While they have some knowledge I see no way they can know the performance  features of the tread rubber or the construction features of the body of the tire that was designed, selected and manufactured for that specific vehicle.

TT While I am not aware of any similar testing and in vehicle evaluation on tires for trailers in the RV world, there are still some requirements the RV Certification Label aka Tire Placard sticker has to meet BY LAW. Tires selection is the responsibility of the RV mfg. The inflation specified on the sticker along with the tire type and size must be capable of supporting at least 50% of the stated GAWR per DOT regulations for each tire. A few years ago RVIA decided that having at least a 10% load capacity margin would be a good thing and that having such a margin should improve tire reliability so the sticker inflation for the specified tires must provide AT LEAST 110% of the GAWR and assumes a perfect 50/50 end to end of each axle oad split. So if your RV is RVIA certified the sticker as applied by the manufacturer must meet these standards.

I think a point of confusion is the wording on some tires concerning the Max Load capacity of a tire and the appropriate inflation needed to provide that load capacity. Each tire has an absolute Maximum Load capacity number and that is the number stated on the tire sidewall. The confusion comes in when people think that they can obtain more load capacity if they increase the inflation but that idea is incorrect once the maximum load capacity of the tire has been reached. While it is possible to increase the inflation doing so will not increase the laod capacity and that is why some tires say "Max" inflation but everyone should know or realize that tires warm up when in the Sun or when driven and that pressure increases with temperature (about 2% per each 10F) BUT when discussing inflation numbers or when setting tire pressure we are ALWAYS talking about the pressure when a tire is at the prevailing Ambient temperature i.e. temperature in the shade and the tire has not been warmed by either being in the Sun or being driven on for the previous 2 hours. i.e. "Cold Pressure".

As an experienced tire design engineer I can only advise that people follow the car company and RV company recommendations for tire inflation unless they follow the specific guidelines provided by actual tire engineers if they want to fine tune tire pressure based on actual scale measurement and include the suggested tolerances and margins on load and inflation.


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