Atombum, that is really not a good thing to do. The purpose of the air pressure setting the car manufactures put in the door jam is the identify the pressure at which your tire will be flat on the road and will wear evenly as you drive. You want the tire to be flat on the road and making even pressure contact with the road all the way across the contact patch. This gives you the best drivability, even wear, and most importantly, the best braking and steering control.
Over pressured will make the tire bulge in the middle making the primary contact patch in the middle of the tire. This makes the car wander and follow cracks and grooves in the road and reduces braking and steering performance.
Under pressure makes the tire sag in the center and causing it to wear fast on the outside edges, causes more flex in the sidewalls which builds heat and can cause a blowout. and poor drivability as well. And it also can reduce braking and steering control.
If you want to adjust tire pressures based on load, do what Powertechn2 says above which is to get the algorithm from the tire manufacturer and adjust it up or down based on the actual tires you have, vehicle weight, loading, etc. Or, do as I do, which is to put the pressure recommended from the door sticker in the tires and watch closely as they wear. If the outside edges are wearing more than the middle of the tire, add a couple psi. If the middle is wearing more than the edges, remove a couple psi. The door sticker is usually pretty close to correct.
If you change tires ratings or sizes, you have to re-figure the correct air pressure. Again, doing what Powertechn2 says is the best way to determine the new pressure. You can also guesstimate it by looking at the max load rating at max pressure on the new tires vs. the old tires to determine if you should run more pressure or less pressure. To get close, use the formula:
(sticker pressure) / [ (max weight rating of stock tires)X(max weight rating of new tires) ] = (new pressure).
This will get you in the ball park. Don't set them below about 27 psi to start out (no matter what the formula says) and watch how they wear and adjust from there. If you are at 27 psi and they are wearing more in the middle, you can reduce the pressure but be careful. You don't want the sidewall flexing too much building too much heat in the tires, especially if you live in a hot area. That is how Ford/Firestone killed some folks by under inflating their tires.
If you put 1500 lbs in the bed of the truck and you are only going a few miles, you don't need to do anything. But if you are going to drive across the US that way, then you might want to add some pressure to the rear tires to compensate for the load.
