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Tire air pressure

When new tires are installed they are typically inflated off the car. The pressure is adjusted, they are bolted on and then the car is lowered to the ground. Why doesn’t the weight of the car increase the tire pressure?

It does make some difference, but not enough to worry about. As the weight of the car is added it just changes the profile of the tyre so the total volume and pressure remains about the same.

pV=nRT

It does - and at worst, it is 1/2 psi.

The real problem is tire growth. Believe it or not, but within the first 24 hours, a tire will grow enough to reduce the pressure up to 3 psi. Some knowledgeable tire shops will over inflate the tires so they can be adjusted downward later - but some overinflate just due to habit without really knowing why they do it.