A tire is comprised of many individual parts that are bonded together through a high-heat-curing process called vulcanization. As with all auto parts, normal wear and tear affect a car part's longevity. When a tire endures normal use, several factors contribute to the tire wearing down and losing its desirable properties. Stress from the weight of the car causes oxygen to be forced against the inside of the tire walls. Heat from a tire's friction against the pavement can also cause a tire to wear down on the inside. The process of heat and oxygen affecting a tire's material properties is called thermo-oxidative aging [source: Safety Research and Strategies, Inc.].
According to the National Highway Transportation Safety Administration (NHTSA), a tire ages when there is a "loss in a tire's material properties, which over time leads to a reduction or performance capabilities" [source: National Highway Traffic Safety Administration]. The NHTSA is conducting more car part information testing with tires to provide the industry and consumers with the most significant information possible.
If a tire is placed on a car shortly after it's been made, the oxidation that occurs has a minimal effect on the tire because the tread usually wears down before the oxidation causes any major problems [source: Safety Research and Strategies, Inc.]. But tires that are mounted on a rim, even if they aren't used or if it's used infrequently, like a spare tire, for example, can still undergo oxidation. Think of it this way; if you buy a gallon of milk but don't open it for 8 weeks, it doesn't mean that milk will still be fresh when you do use it. The milk will curdle regardless of whether or not you've opened it. The same goes for tires. Having a tire sitting around for several years may actually reduce its longevity.
Not only does oxygen break down a tire's material properties, but heat also causes tires to break down even faster [source: Gin]. The NHTSA reported that from 2002 to 2006 a large insurance company said that 77 percent of its tire claims came from five states with hot ambient temperatures. In addition, 85 percent of those claims were for tires that were more than six years old [source: National Highway Traffic Safety Administration].
With tires that are aging before they're even in use, consumers could be purchasing sets of tires that may not be as long lasting as the mileage they're guaranteed for. But is this really a problem?
Up next, find out how a tire's age could be dangerous for drivers.