I have a Truck that has a recommended tire pressure of around 30 psi on all four tires. On the front two tires I got about 30 psi, but on the back tires I've got 40 psi. The sidewall maximum pressure reads 35 psi. The thing is, I put a lot of stuff in the trunk, and sometimes i measure my tire pressure and it goes all the way up to 70 psi. So my question is, since I always carry such a heavy load, should I lower my current 40 psi, or should I just leave it there so my tires won't be super flat when I carry heavy stuff?
The tire manufacturer and the truck manufacturer both recommend tire pressures. The tire manufacturer will have it printed on the side wall, while the truck manufacturer will have it printed on the inside of the drivers door pillar or just refer to the owners manual.As long as you are going with the size tire that the truck manufacturer recommends,use the tire pressure that they print.That tire that they recommend is also the tire that corresponds to the allowable weight that the truck will haul.If you are using a tire other than the truck manufacturer allows,use the pressure printed on the tire.
If you load down the truck then you need to buy some heavy ply tires. Most Light Truck tires are 6 - 8 ply. You can purchase some 10 ply tires that run around 65 to 80 psi and several sizes. Most trucks that are used for heavy loads have 10 ply tires.
Lower your tire pressure to the factory recommended pressure. The higher your tire pressure the higher the chance to have a blowout, and the worse tread damage you do to the tire, which also increases the chance of having a tire failure. When you tire pressure is too high then the inside of your tire tread suffers more wear, when your tire is under inflated then the outside of the tire tread will suffer more damage. When you increase the weight in the truck you are increasing the pressure in your tires. All this damage is cumulative and eventually it will create a tire failure. You have been pushing your luck for a while now. It is true that you get better gas mileage with higher tire pressures, but it isn't worth the risk of having to repair or replace the tire and side blowouts cannot be fixed. Another danger you are suffering that you don't realize is that you are putting less tire tread on the road and so you have less control, in a rainy or other slick road condition you have a greater chance of losing control and causing an accident. Since you have been doing this for a while your chance of getting into an accident is only increasing.
If what you load affects the inner pressure of your tires, then the tires on your truck are likely not rated for the kind of load you are hauling, meaning you probably need a higher load rated tire because what you're doing could result in a blowout. I am not sure what kind of truck you have, but on my D-2500 3/4 ton pickup truck I have the kind of tires that require 50psi per tire, and they make even heavier duty tires that require 80 psi per each, and so on. As a rule, 4 tires for my ride cost $500 for starters, and from the sound of things you have car tires on yours, and you need truck tires.
The tire pressure ratings molded into the sidewall of a tire are the maximum inflation rating "COLD" of that tire at the maximum weight listed on the sidewall. This DOES NOT reflect what the tire may "run" at in operation on the highway. NEVER exceed the maximum pressure or the load rating listed on the sidewall. Yes the tire is capable of handling more than listed but do you really what to bet your life on how much more? If you require more load capability in your tires you will have to pony up for a better rated tire. Just remember, never exceed the vehicles maximum GVWR no matter what the tires are good for. Good Luck
