October 2007

---

Dear Tom and Ray:

Is there any way the tire pressure can increase without manually filling the tire? I was driving from Flagstaff, Ariz., to Tucson, and 30 minutes from Tucson, the tread blew off the front driver’s side tire. The tire guy tested the pressure, and said someone must have put air in the tire during an oil change or something. But it had been several months since any service. Can altitude, barometric pressure, heat or speed affect the tire? It is a 1991 Honda Accord with 198,000 miles — just because I knew you’d want to know. — Bryan

TOM: You’re a lucky guy, Bryan. Often, when one of the belts blows off, the whole tire comes apart. Including the air. And at highway speed, that can be extremely exciting!

RAY: To answer your question, over-inflation probably had nothing to do with this. It had more to do with the age and condition of your tires.

TOM: To get more details, we checked with the Quincy, M.E. of the tire world, Bill Woehrle.

RAY: Bill spent his career as a tire engineer, and now runs a company called TFI: Tire Forensics Investigation — which we expect to see on CBS next fall.

TOM: Bill says that over-inflation almost never causes tire failure. The standard tire is inflated to about 30 to 35 pounds per square inch. Under hot weather and highway conditions, the temperature of the air inside the tire rises about 50 degrees. That increases the pressure inside the tire about 5 psi. The burst pressure of a tire is about 200 psi. So unless you had your tires pumped up to 195 psi (trust us, you didn’t), you didn’t come anywhere near bursting the tire from too much internal pressure.

RAY: Bill says that the most vulnerable part of any steel-belted radial tire is where the steel belts are attached to the rubber near the edges of the tread, also called the “shoulders” of the tire. If the tire is not abused, those belts should stay attached to the rubber for the entire tread life of the tire.

TOM: But if the tire is defective (see Firestone debacle) at the end of its useful life (and since you drive a car with 198,000 miles on it, Bryan, we can’t help but wonder if those tires are older than the polyester leisure suits in my brother’s closet), or if the tire has been abused in some way, the top belt can separate. When it tears, it tears violently, so it’s a crapshoot as to whether the next layer of rubber will tear too and cause a blowout.

RAY: And the most common form of abuse is under-inflating your tires. That’s right, UNDER-inflating them.

TOM: Let’s go back to our forensic tireologist. Woehrle says that under-inflating the tire puts additional stress on the tire’s shoulders, where the belts are attached. And if a tire is under-inflated by 10 or 15 psi, the temperature at those shoulders can reach 200 degrees. So you’ve got a vulnerable part of the rubber that’s hot being stretched and pulled, and that’s a recipe for tire failure.
RAY: By the way, overloading the car with say, luggage or two mothers-in-law does the same thing as under-inflating the tires. It stresses the shoulders of the tires.

TOM: So Bill says if you’re going to make a mistake in inflating your tires, it’s much better to over-inflate them a bit than under-inflate them. There are really no horrible ramifications of over-inflating the tires by 10 or 15 psi, except that when you go over bumps you’ll bounce up and hit your head on the dome light.

RAY: But the dangers of under-inflation are catastrophic tire failure. So for all of our readers: Make sure your tires have sufficient tread and have not exceeded the manufacturer’s mileage rating. Make sure they’re less than 10 years old, even if they have low miles (because old rubber gets brittle and has a greater tendency to crack). And check your tire pressure at least once a month to make sure your tires aren’t under-inflated.

Back to the October 2007 index

Share this column with a friend.

© 2008 by Tom and Ray Magliozzi and Doug Berman. Distributed by King Features Syndicate, Inc.

---

Auto repairs can be costly! Save money by ordering Tom and Ray's pamphlet: "Ten Ways You May Be Ruining Your Car Without Even Knowing It!" To order, send (check or money order) to Ruin, P.O. Box 536475, Orlando, FL 32853-6475. You can also order online.