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According to Edmunds.com, 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.

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?
Carmakers, 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 carmakers and tire manufacturers. Carmakers 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.

The Rubber Manufacturers Association says there is no way to put a date on when a tire "expires," because such factors as heat, storage and conditions of use can dramatically reduce the life of a tire. Here's more on each of these factors.

Heat: NHTSA research has found that tires age more quickly in warmer climates. NHTSA also found that environmental conditions like exposure to sunlight and coastal climates can hasten the aging process. People who live in warm weather and coastal states should keep this in mind when deciding whether they should retire a tire.

Storage: This applies to spare tires and tires that are sitting in a garage or shop. Consider how a spare tire lives its life. If you own a truck, the spare may be mounted underneath the vehicle, exposed to dirt and the elements.

If your spare is in the trunk, it's as if it is "baking in a miniature oven," says Dan Zielinski, senior vice president of public affairs for the Rubber Manufacturers Association. Most often, the spare never sees the light of day. But if the tire has been inflated and mounted on a wheel, it is technically "in service," even if it's never been used, Gervin says.

A tire that has not been mounted and is just sitting in a tire shop or your garage will age more slowly than one that has been put into service on a car. But it ages nonetheless.

Conditions of use: This refers to how the tire is treated. Is it properly inflated? Has it hit the curb too many times? Has it ever been repaired for a puncture? Tires on a car that's only driven on the weekends will have a different aging pattern than those on a car that's driven daily on the highway. All these factors contribute to how quickly or slowly a tire wears out.

Proper maintenance is the best thing a person can do to ensure a long tire life. Gervin recommends that you maintain proper air pressure in tires, have them rotated regularly and have them routinely inspected.

How To Determine the Age of a Tire
The sidewall of a tire is full of numbers and letters. They all mean something, but deciphering them can be a challenge. To determine the age of a tire, you'll just need to know its U.S. Department of Transportation (DOT) number.

Tires made after 2000 have a four-digit DOT code. The first two numbers represent the week in which the tire was made. The second two represent the year. A tire with a DOT code of 1109 was made in the 11th week of 2009. Tires with a three-digit code were made prior to 2000 and are trickier to decode. The first two digits still tell you the week, but the third digit tells you the year in the decade that it was created. The hard part is knowing what decade that was. Some tires made in the 1990s (but not all) have a triangle after the DOT code, denoting that decade. But for tires without that, a code of "328" could be from the 32nd week of 1988 — or 1978.

Clearly, these DOT numbers weren't designed with the consumer in mind. They were originally put on tires to make it easier for NHTSA to recall tires and keep track of their manufacturing date.

To make matters worse, you might not always find the DOT number on the outer side of the tire. Because of the way a tire is made, it is actually safer for the technician operating the mold to imprint information on the inner side of the tire, so some manufacturers will opt to put the number there. It is still possible to check the DOT code, but you might have to jack the car up to see it.

That potential inconvenience is going away, however. NHTSA says that the sidewall information about the tire's date of manufacture, size and other data is now required to be on both sides of the tire for easier reading.

After checking out a tire's birth date, give the rubber a visual inspection. Some of the best advice on such an inspection comes from the British tire manufacturers association. It recommends that consumers check tires regularly for any sign of aging, such as tread distortion or large or small hairline cracks in the sidewall. Vibrations or a change in the dynamic properties of the tire could also be an indicator of aging problems, the association says. It recommends replacing the tire immediately if such symptoms appear.

Don't Buy Used
Tires are expensive, especially when you factor in the price of mounting and balancing. That's why used tires become more attractive to consumers who are strapped for cash. But the purchase of used tires is very much a buyer-beware situation, Zielinski says. "Even a one-year-old tire can be dangerous if it was poorly maintained," he says.

When a consumer buys a used tire, he has no idea how well it was maintained or the conditions in which it has been used. The previous owner might have driven it with low pressure. It could have hit curbs repeatedly. It could have been patched for a nail. Further, it's a dated product.

"You wouldn't want a used tire for the same reason that you wouldn't buy a 10-year-old computer," Zielinski says. "You are denying yourself the advancements in tire technology over the past few years."

Letting Go
Getting rid of an unused spare or a tire with good-looking tread may be the hardest thing for a thrifty consumer to do. "Nobody's going to take a tire that looks like it's never been used and throw it out," Kane says. But if it's old, that's exactly what the owner should do.

Although Kane has lobbied NHTSA to enact regulations on tire aging, nothing is currently on the books. A NHTSA spokesman says the organization is "continuing to conduct research into the effects of tire aging, and what actions consumers can do to safely monitor their tires when they are on their vehicles."

It's too bad that tires don't have a "sell by" date, like cartons of milk. Since there's no consensus from government or industry sources, we'll just say that if your tire has plenty of tread left but is nearing the five-year mark, it's time to get it inspected for signs of aging.

Of all your vehicle's components, tires have the greatest effect on the way it handles and brakes. So if the tire store recommends new tires at your five-year check-up, spend the money and don't put it off. Your life could depend on it.

Finish Line Tire and Automotive can help you stay safe with regular tire inspections when you have your vehicle serviced with us. We inspect your tires and other vital vehicle systems to insure you are aware of any potential issues. And, when it is time we can help you choose a new tire which best fits your vehicle and driving habits.