Originally Posted by canadianelbow
So lets assume for a moment that ford recomends 33 psi on the door jamb on a '10 f150. (I'm too lazy to go look at the exact #). Are you saying that they make that recomendation based on the specific tire they chose for that model? And if you change tires to say the nitto trail grappler 35/12.5/18lt, that you would have to ignore the door jamb tire pressure and go with the recomended pressure from nitto? Where would you even find a recomended pressure from the manufacturer? They only print the max tire pressures on tires, not recomended. Max, and recomended are not the same......
Yes, it is for that specific tire. You will find different recommended pressures on the sticker depending upon which tire came with the vehicle. As an example, mine came with a LR C LT tire. Recommended pressure is 45. Yes if you change tires you should ignore the door sticker unless, it is the same LR, weight carrying ability, and max pressure.
Unfortunately, there is nowhere to go to get what you should use. I have heard of tire dealers that say just use what is on the door for a LR E tire with a max pressure of 80, when the door sticker was for a P series with a max of 35. 35 is way too low for a LR E with that max. At best, a LR E, which has a very high weight rating, 40 would be the minimum, and then only for an unloaded vehicle. A LR E will normally have a weight rating from 3200 to 4000lbs. At 35, the tire cannot handle half of that value, and could be dangerous on the highway. You just have to use some common sense and look at the numbers.
Some other place to look at is the rule NHTSA created for the use of the Tire Pressure Monitoring Systems.
Scroll down about 1/4 of the way on this. It has a chart of pressures. Even though 46 is listed as the minimum for a LR E, some tires have a much higher rating, so that when used on an F150, lower than 46 could be okay, but anything lower than 40 could be a problem.