Tires: Truck Tire INflation, truck tire inflation, tire placard

Question
I own a 93 F150 4WD.  The tire placard says use LT235/75 15 tires recommending 35 PSI front, 41 PSI rear.  It currently has 31.5x 11.5 15s on it.  What is the equivilant inflation pressure I should use for these larger tires?  The truck commonly carries moderate loads, very rarely close to the weight limit.  Thanks

Answer
Robert,

You should look at the door placard again, because my book says the tire size Is "P" not "LT".

But there is a problem that you may be unaware of:  According to Tire Guides, the stock rims on a 1993 Ford F-150 are 6" wide and a 31X11.50R15LT requires a minimum of an 8" rim.  

There is something about what tire Guides says that doesn't ring true.  I suspect Tire Guides is only quoting the steel rim, and I'll bet that F-150's sometimes came with 7" rims.  However, I don't think Ford ever offered anything larger until fairly recently.  You should check the vehicle placard because I think the rim size is listed there as well.

What thsi means is that the tire is going to "arch" across the tread face and that might result in wear in the center of the tread and a tendency to wander.

But to answer your question, the equivalent pressure would be 36 psi and 40.