In February 2008, the owner of a 1998 Ford Explorer in Georgia needed a new tire for his SUV and ended up buying a used one. When he was driving two weeks later, the tread suddenly separated from the tire. The Explorer went out of control and hit a motorcycle, killing its rider. An analysis of the used tire revealed that it was nearly 10 years old.

More recently, an investigation into the cause of the accident that killed the actor Paul Walker revealed that the Porsche Carrera GT in which he was riding had nine-year-old tires. The California Highway Patrol noted that the tires' age might have compromised their drivability and handling characteristics, according to the Los Angeles Times.

These incidents illustrate not only the potential danger of buying used tires but also the perils of driving on aging tires — including those that have never spent a day on the road.

For years, people have relied on a tire's tread depth to determine its condition. But the rubber compounds in a tire deteriorate with time, regardless of the condition of the tread. An old tire poses a safety hazard.

For some people, old tires might never be an issue. If you drive a typical number of miles, somewhere around 12,000-15,000 miles annually, a tire's tread will wear out in three to four years, long before the rubber compound does. But if you only drive 6,000 miles a year, or have a car that you only drive on weekends, aging tires could be an issue. The age warning also applies to spare tires and "new" tires that have never been used but are old.

What Happens to a Tire as It Ages?
Sean Kane, president of Safety Research & Strategies, Inc., compares an aging tire to an old rubber band. "If you take a rubber band that's been sitting around a long time and stretch it, you will start to see cracks in the rubber," says Kane, whose organization is involved in research, analysis and advocacy on safety matters for the public and clients including attorneys, engineering firms, supplier companies, media and government.

That's essentially what happens to a tire that's put on a vehicle and driven. Cracks in the rubber begin to develop over time. They may appear on the surface and inside the tire as well. This cracking can eventually cause the steel belts in the tread to separate from the rest of the tire. An animation on the Safety Research & Strategies Web site shows how this happens. Improper maintenance and heat accelerate the process.

Every tire that's on the road long enough will succumb to age. Tires that are rated for higher mileage have "anti-ozinant" chemical compounds built into the rubber that will slow the aging process, but nothing stops the effects of time on rubber, says Doug Gervin, Michelin's director of product marketing for passenger cars and light trucks.

How Long Does a Tire Last?
Car makers, tire makers and rubber manufacturers differ in their opinions about the lifespan of a tire. The National Highway Traffic Safety Administration (NHTSA) has no specific guidelines on tire aging and defers to the recommendations of car makers and tire manufacturers. Car makers such as Nissan and Mercedes-Benz tell consumers to replace tires six years after their production date, regardless of tread life. Tire manufacturers such as Continental and Michelin say a tire can last up to 10 years, provided you get annual tire inspections after the fifth year.