Question for all you physicists out there.

When the temperature outside drops, will the pressure in your tires increase or decrease?

When a gas is heated it expands, right? So does that mean that when the temperature outside goes up, the air molecules in the tires should expand as well causing tire pressure to go up?

Then when the temperature outside goes down the opposite happens, tire pressure will decrease.

Is my thinking correct, or am I way off?

 

You are correct.

This is why you are supposed to regularly check tire pressure before driving the car. As the tire is used, its temperature increases, and so does the pressure. Pressure is specified under cold conditions.

When winter rolls around, you might need to add some air as the average temp drops. As summer comes in, you might need to bleed some off. This is assuming you somehow have perfect sealing on your tires, of course.

This is also why you see autocrossers bleeding off pressure from all tires between each run, to maintain the correct "hot" pressure. If I first grid with 32 psi up front, I might come back from the first run with 36 psi, bleed back to 32, do 2nd run, and be back at 36 again, bleed back to 32 for 3rd run, and find them at 20 psi once they're cold again.

 

Yup. From high school physics, PV=nRT. Temperature goes up, pressure goes up.

The Tire Rack website says that a rule of thumb is a difference in temperature of 10 degrees F corresponds to 1 psi.

 

and nitrogen is an option.