RAY MAGLIOZZI

Dear Car Talk: I do volunteer trail work in the Cascades with a crew. Yesterday, my tire pressure light went on while I was going out to work. I pulled off the road and looked at everything and neither heard nor saw any obvious damage or leak. Other people, in better vehicles, assured me they had an air compressor, and we were only 4 miles from the work site, so we kept going.

I parked the car and we worked on the trail for several hours. When I returned, the left rear tire was flat. One of my co-workers put his portable compressor on it and started it up. It was 5.5 psi to begin with, and we got it up to 33.5.

At this point, I am 40 miles from a tire shop, and I decided to drive it. My colleague gave me the air compressor to take with me. Within 5 miles, the tire pressure light came back on, but I made it to the tire shop, and the tire didn’t look too bad when I stopped.

But this question was asked by several crew members: When you drive on a leaking tire, does it leak more, less, or the same, assuming it is not a sidewall blowout? Does the warming air inside have any effect? — Mike

Well, as excited as I am to make up an answer for you, Mike, I instead called on Car Talk’s resident professor of physics, Wolfgang Rueckner. Wolf used to be employed by Harvard University. Until they caught him advising Car Talk.

Anyway, Wolf says that the tire would most likely leak faster as the car was driven. Here’s why: As you suggest, when the tire heats up, due to flexing and road friction, the air inside the tire also heats up. According to a bunch of formulas that Wolf sent me on the back of a Voodoo Donuts napkin, raising the temperature inside a confined space like a tire increases the air pressure in there, too. And the higher the pressure, the more the air wants to get out.

Now, if your leak is caused by, say, a nail or screw hole, the hole might expand a bit too as the pressure inside the tire increases, but Wolf says that wouldn’t be a major factor.

The only scenario in which the tire might leak less is one in which the leak is caused by a poor seal of the tire’s bead. The bead is the part of the tire that pushes out against the inside of the rim and seals the air in. In that case, increased pressure from within could actually improve the seal and cause the tire to leak more slowly.

But, in most cases, a leaking tire will leak faster when you drive on it. And in your case, we’re grateful it just didn’t leak fast enough to strand you, Mike.

Got a question about cars? Write to Ray in care of King Features, 300 W. 57th St., 41st Floor, New York, NY 10019, or email by visiting the Car Talk website at www.cartalk.com.