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Tires/Tire pressure


QUESTION: I recently changed the OEM tires on my Toyota Tunrda.  The "P" rated tires was replaced with a larger "LT" rated tire.  The Tire pressure was set to 55PSI by the Tire Shop.  All seemed fine for about a year or so until I realized that at an oil change performed by Jiffy Lube. The technician reduced the air pressure to 32PSI unknown to me until I checked it myself about six months later.  I checked the tires because they looked low and I was experiencing some shimmy in my steering wheel more so at 60 mph and higher.  I increased the air pressure to 60 PSI and the shimmy is reduced by 80%.  Can the air pressure of a tire cause "cupping" or shimmy in the steering wheel and cause a tire to become "out of round?"

ANSWER: Jerard,

By itself reduced inflation pressure does not CAUSE cupping wear in a tire.  HOWEVER, reduced inflation pressure increases the effect those things that cause cupping wear have.  For example, a properly aligned, but low inflated tire doesn't result in irregular wear, but a mis-aligmed properly inflated tire will, AND, low inflation pressure makes that situation worse.

---------- FOLLOW-UP ----------

QUESTION: Thank you for your quick response.  I do have a follow up question.  I realize that the decal on the drivers door illustrates the proper air pressure for tires.  With that said, is that information only good for the tires originally installed?  I see in this forum that it is highly discourage to use the recommended air pressure as illustrated on the tire but I am not using the original tires and am using a larger "LT" tire and not a "P" tire.  The operating air pressure on the LT tire is much higher (of course if a heavy load is being hauled) than that of the "P" tire.  In my post above I mentioned filling the tires to 60 PSI.  In your opinion, can I safely operate the vehicle at this pressure without a load?  Also, if I frequently have a load in the bed do I constantly have to service and deservice the tires or can I just keep the pressure at 60 PSI?


The vehicle tire placard lists the original tire size and the proper pressure for that SIZE - which includes the letters immediately proceeding or following the numbers in the size.  In your case, you have LT tire in place of P type tires, and those 2 types have enough differences that even if the numbers are identical, the proper inflation pressure is different - by 15 psi (LT needing higher pressure!)

Also, it doesn't matter what brand or model tire is used, if the size is the same, then the proper pressure is also the same.

I strongly suspect that 60 psi is wrong for your vehicle.  If you'll do another follow up post with the tire size listed on the vehicle tire placard, the inflation pressure listed there, and the size you are actually using, I'll do the calculation.  


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Barry Smith


I have over 30 years experience in the design, manufacturing, and testing of tires. I have served as the technical advisor to the "800" number. I have authored or co-authored many publications - usually without credit. I can answer almost any technical question, but please don`t ask me to compare brands. I probably have prejudices because of my work experience.


Member SAE (Society of Automotive Engineers) Member Tire Society (Tire Technical Organization) SCCA Regional Competiton License holder Authored many training manuals on tires, their care and use.

SAE (Society of Automotive Engineers) SCCA (Sports Car Club of America) Tire Society

You may want to visit my web site:

I am a graduate of the University of Dayton, Dayton, OH - BME, 1971 I attended graduate school at the University of Akron - but did not complete my PhD - something I regret.

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