Tires: Tire pressure, proper rotations, alignment settings

Question
Barry,
I own a 2003 chevy silverado ext cab 4x4 my manufactures
recommends a front and rear 35 psi pressure on P265/75R16.
I have a 6” lift and have a LT315/75R16, I have been running
them at 35psi and religiously been maintaining proper
rotations and maintenance, and the place I bought them
states they are beginning to ‘cup’.  I have had them road
force balanced, and the vehicles alignment checked, with
which is all ok.
What pressure should the tires be set at.
Thank you for your time.
Steve


Answer
Steve,

In order to carry the same load as a P265/75R16 does at 35 psi, an LT315/75R16 needs to use 31 psi.

But cupping is usually caused by mis-alignment and inflation pressure is just an aggravating condition.  So you need to look at the alignment.

My experience says that the published alignment tolerances are too wide.  Not the target value, but the allowable deviation from that value.  I think it ought to be half of what is published.

Put another way, the alignment should be within the inner half of the spec.

You should be aware that even vehicles that do not have a pull can be out of alignment.  There are settings where one out of spec condition is offset by another out of spec condition – typically camber vs toe.  

Also, many alignment techs think that if the factory did not make provisions to make adjustments for the alignment, then they can’t make an adjustment and will declare the vehicle “OK”.  This is totally wrong.

ALL alignment settings are adjustable, but it may require an eccentric bolt, some shims, etc.  A GOOD alignment tech will know what to do and the vehicle should leave a shop with ALL the alignment settings close to the nominal.

So when you get your next alignment be sure to ask to have the setting put into the inner half of the spec, and get a "before" and "after" printout.  If the shop will not do that - find another shop.

And while you are at it - you are aware that the stock wheels for your truck were 7", and an LT315/75R16 requires a rim width range of 8" to 10", so if you didn't change rims, this would also aggravate the situation.