Originally Posted By: Snoman002
Just remember, the factory sticker on the door jam is for factory tires with a factory load range. It's highly unlikely there "best" pressure would be wildly different in this case, but something to be aware of.
Personally I'm a fan of measuring the contact patch to determine proper pressure.
A couple of thoughts:
1) The vehicle tire placard will list the original tire size and the specified pressure for that SIZE. Please note that for passenger cars, this pressure would be the same REGARDLESS of who manufactures the tire. That's because tires are standardized. I go into this in much greater detail here:
Barry's Tire Tech - Load Tables
For pickup trucks and vans, the situation is a bit more complex. P type tires are load rated one way, and LT tires are rated another way. So if your pickup or van came with P type tires and you want to put on LT tires, you will need to adjust the pressure.
If your vehicle came with LT tires, then you can NOT safely put on P type tires.
2) Measuring the contact patch is NOT a good way to decide what the inflation pressure should be. I do not have any idea HOW that could be done, but ALL the tire manufacturers use the load vs pressure charts to test the durability of their products - and NONE of them use the contact patch size or shape as a determining factor. They might measure the contact patch in order to provide a better wearing or better handling product, but that is a consequence of the design, not a criteria for durability or ride.
3) The Ford Explorer situation a few years back: I go into that in detail here:
Barry's Tire Tech - The Ford/Firestone Controversy
Short version: While Ford's spec for the inflation pressure for the Explorer was not the best, it wasn't the real problem. Ford, GM, Dodge, all specified pressures the same way - the load carrying capacity of the tires = GAWR (Gross Axle Weight Rating). Not good engineering practice - BUT - they all did it, and only the Ford Explorer with the Firestone tires had issues. Even Ford Explorers with other brands did not have issues.
The problem was a combination of a quirky tread pattern and an odd manufacturing technique. Both of those things have been identified by both Firestone and other tire manufacturers and appropriate changes have been made. The fact that Ford increased the inflation pressure specification is more a reflection of recognizing there was a BETTER way to specify the load carrying capacity of the tires.