Tires: tire pressure, tire failure, sway bars

Question
I have a 2011 F-150.  Came with LT245/75R17, but rough ride so dealer put on P265/70R17.  Plate on inside of door calls for LT245/75R17 with 55 psi.  Just recently put on LT265/70R17 as they look better than the 245's and wanted to have LT's as door sticker said.  Question - what is the minimum psi I can carry to get the best ride.  Will I sacrifice mileage and/or tire ware with to little psi?  Typically, haul air.

Answer
Martin,

I am not a fan of replacing P metric tires with LT metric tires and vice versa - and here's why.  

When your dealer replaced the LT245/75R17's which should use 55 psi, the P265/70R17's needed to use 34 psi to carry the same load.  That is a difference of 20 psi - which changes the spring rate of the tire and affects the way the vehicle handles.  Ford spent a lot of time doing handling maneuvers in order to select the springs, shocks, and sway bars so the vehicle behaves predictably.  Changing the spring rate of the tire might have resulted in a vehicle that didn't behave predictably - and usually the first time that is noticed is in an emergency situation when you need it the most.

Now you want to put LT265/70R17's on and that means you should use 50 psi.  Yes, you will sacrifice mileage and wear with too little psi - and worse, you will increase the risk of a tire failure which sometimes has tragic results.