There’s More to Tire Safety Than Tread Life and Mileage

How many miles can you expect to drive on a set of tires?

I have a set of tires on my car that seem as if they’ll never wear out. Even tire experts are perplexed by how a set of tires could last so long.

In most cases, motorists complain that their tires wear out prematurely, going bald long before the warranties that cover them expire. But my case is exactly the opposite. I’m wondering whether there’s anything inherently dangerous about continuing to use tires after so many thousands of miles--even if there is adequate tread.

How many miles are on my tires? I’ve been rumbling down bumpy Southern California freeways and along the potholed parkways of Washington, D.C., with my original tires for more than 127,000 miles. Even though they were clearly premium tires, it seems crazy even to me that they are still so sound that they rarely need air.

“Every time I mention 127,000 miles, our [safety] people raise their eyebrows,” said Jeff Spring, a spokesman with the Automobile Club of Southern California.

“Nobody has ever heard of that many miles on tires,” according to a tire industry expert.

The purpose of this column isn’t to plug the tire company that made my tires, because I’m not going to mention it. But I do want readers to better understand the difference between a safe tire and an unsafe tire.

Tires are supposed to be replaced if they have less than three-thirty-seconds of an inch of tread remaining. Under that rule, my tires are still OK. Most people don’t have or can’t read a ruler with 32nd marks. A rule of thumb is that you should place a penny upside-down between the treads, and if Lincoln’s head falls below the edge of the tread, you’re safe.

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Assuming you have enough tread, what can go wrong with a tire? Plenty. After many years of use, a tire’s sidewall can develop cracks in the rubber from ozone damage, according to the Rubber Manufacturers Assn., a trade group. This is called checking. If the cracks are deep enough, they can weaken the sidewall. Or they can allow water to penetrate into the cord body and cause a belt separation.

Tire belt separation sometimes causes a thumping sound. Driving on separated belts for even one day is bad judgment. Another serious problem as tires wear is gashes caused by glass or other road hazards. You want to make sure the cuts don’t extend down into the cord body.

As tires age, it’s not unusual to see a tire bead, the edge that seals against the wheel rim, weaken or leak. In fact, most leakage occurs around the tire bead. There isn’t a visual check for this problem, but monitoring tire pressure should give you a good idea of whether you have leakage.

Despite my tires’ age, they do not leak very much, and there are no signs of serious checking.

Tire companies love to give general advice about tires, but it is often cautiously phrased to avoid liability lawsuits. The maker of my tires, for example, would not say if there is any expected life limit to tires. Its only advice was to have my tires inspected by a “professional” for potential structural damage.

Another standard caution is to maintain correct air pressure, but no manufacturer ever provides advice on how much leakage is normal or acceptable. Is a leak of 5 pounds per square inch a month normal? Don’t ask the rubber industry.

Why have my tires lasted so long? It’s mostly luck. The tires are relatively big for my vehicle, so they don’t wear as fast. I obviously don’t burn rubber on my starts. And I occasionally check my pressure, but I’m not religious about it. I think I’ll shoot for 130,000 miles.

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Ralph Vartabedian cannot answer mail personally but responds in this column to automotive questions of general interest. Write to Your Wheels, Business Section, Los Angeles Times, Times Mirror Square, Los Angeles, CA 90053.

Via e-mail: ralph.vartabedian@latimes.com.