Here is an informative article from edmunds.com
Do Car Tires Have a Shelf Life?
Ronald Montoya
by
October 24th, 2018
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For years, people have relied on tread depth to determine when to replace a tire. If the tread passes the "penny test," they assume the tire still has life, regardless of how old it is, which can be a fatal mistake. Old tires are dangerous, regardless of tread depth. While there's no federally sanctioned safety guidance on when a tire is too old to be safe, many carmakers recommend replacement at six years from the date of manufacture.
Old tires have been the culprit in fatal accidents. Here are just two examples:
In 2008, the owner of a 1998 Ford Explorer in Georgia needed a new tire for his SUV and bought 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.
In a more recent and high-profile example, the investigation into the cause of the 2013 accident that killed the actor Paul Walker revealed that the Porsche Carrera GT in which he was riding had 9-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 the potential danger of buying used tires and the perils of driving on aging tires — including those that have never spent a day on the road. The rubber compounds in a tire deteriorate with time, regardless of the condition of the tread.
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 drive much less than that, or have a car that you only drive on weekends, aging tires could be an issue.
Similarly, if you are buying a used car, there's a chance it may be riding on old tires. The age warning also applies to spare tires and seemingly new tires that have never been used but were produced years ago.
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," said 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.
Cracks in a tire's rubber begin to develop over time, appearing 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. Improper maintenance and heat accelerate the process.
Kane and his organization have identified over 250 incidents in which tires older than six years have experienced tread and belt separations — most resulting in loss-of-control and rollover crashes. These incidents were the cause of 233 fatalities and 300 injuries in 2012.
Every tire that's on the road long enough will succumb to age. Tires that are rated for higher mileage have antiozonant chemical compounds built into the rubber that will slow the aging process, but nothing stops the effects of time on rubber.
How Long Does a Tire Last?
Carmakers, tiremakers 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.
Many automakers, including Ford, Nissan and Mercedes-Benz, tell owners 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 said there is no way to put a date on when a tire "expires" because such factors as heat, storage, underinflation 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, such as exposure to sunlight and coastal climates, can hasten the aging process. People who live in coastal states and other areas with warm weather 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. 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.
Spares: They usually don't see the light of day, but they're still degrading with time. If the tire has been inflated and mounted on a wheel, it is considered to be "in service," even if it's never been used. And if a truck's spare is mounted underneath the vehicle, it's exposed to heat, dirt and weather — all reasons to plan on replacement.
