tire pressure

I'm curious if anything has been written or reported about recommended tire pressure after the vehicle has traveled several miles. Do auto manufacturers factor in distance traveled when they recommend a specific air pressure for tires?

I know that tire pressure should be checked when the tires are relatively cold (driven a mile is OK). I drove about 7 miles today and checked my air pressure. It was at the manufacturer's recommended pressure. So, am I to assume that they are actually underinflated and should have more air?

Do the tires heat up uniformly--i.e. can I check them at home when they're cold, then travel the 7 miles to fill them? Can I assume that if they are 4 lbs. under at home, they still need 4 lbs. when I get to the gas station? If I do that, then they will be overinflated. I don't see this being a tremendous problem now, but come summer when the air temp is 108, it will be.

BTW, I am a cheap-a$$ and this station is the closest with free air to us (and it's on the way to a place we regularly visit, so we're not going out of our way). Everyone else charges at least $0.50 and we usually have to pay $1 to get all tires filled correctly.

Thanks!

Heather

Hi Heather,

Sorry for the very late response. For some reason, the main page did not indicate that there was a new post in this forum.

I generally make sure the tire is at the pressure listed by the manufacturer before I drive it. This assumes your owner's manual says to measure while "cold" before the vehicle has been driven. I have a nice bike pump, which isn't too bad to add a pound or two:-)

It should also work exactly as you said. You just measure at home before you drive. Then add the same amount you would have added at home.

Say the recommended pressure is 35 psi. If it reads 30 psi at home before you drive, you need to add 5 pounds. Even if it reads 32 pounds by the time you get to the station, you will add 5 pounds to make it 37. Just make sure you measure before you go.

The geek formula is Pressure * Volume = n * R * Temperature. Volume, n and R are essentially constant for a tire. That means pressure goes up linearly as temperature goes up. If you plug in the numbers, you actually find that when the tires get "cold" again, you may still have a bit less than 35 pounds in the tire, but it's very close, even for the difference of 77 degrees F to 108 degrees F.

Please don't blame my math if your tires explode:-(

I just looked tire pressure up in our Saturn manual for DH, and it actually tells what to do when the tires are not cold.

"Cold" means your car has been sitting for at least three hours or driven no more than 1 mile. If you have to check tires when they're not cold, ad 4 psi to the numbers on the sticker.

The sticker is the one on the car that tells the correct tire pressure.