How Old — and Dangerous — Are Your Tires?

For all the marvel of today's car safety systems — the networks of cameras, sensors and processors that help minimize driver error — the truth is that a car is still only as safe as the rubber that holds it to the road. That's why it's imperative to replace tires not only when the tread is worn but also when they're old, and even when it appears that plenty of life remains. Conventional wisdom says that if a tire passes the "penny test" (Lincoln's head is fully or partially covered by tread blocks when placing a penny upside down in the grooves), then it still has useful tread life. But this can be a dangerous mistake.

Like any rubber product, tires degrade with age. Regardless of tread depth, old tires can develop cracks and fissures that can lead to tread separation and loss of car control. While there's no hard-and-fast rule, and no federally mandated safety guidance on when a tire should be (ahem) retired, many carmakers and experts advise replacement between five and six years from the manufacturing date.

How dangerous are old tires? Here are a few examples:

• In 2006, an 11-year-old boy was killed when his family's SUV rolled over on a California highway after the left rear tire's tread separated. An investigation revealed that a tire service center had installed the SUV's 12-year-old spare tire during a visit to replace the rear tires.

• 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 driver lost control and hit a motorcycle, killing its rider. An analysis of the used tire revealed that it was nearly 10 years old.

• In 2013, actor Paul Walker died in an accident in a Porsche Carrera GT, one of the highest-performance models in the automaker's history. An investigation determined that the Porsche was driving on 9-year-old tires and traveling at speeds between 80 and 93 mph when the driver, Roger Rodas, lost control and collided with a power pole and several trees. Rodas was also killed.

These incidents illustrate the potential danger of driving on aged and used tires. For many drivers, old tires may never be an issue. If you drive 12,000-15,000 miles annually, a common amount for many commuters, a tire's tread will wear out in three to four years, well 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. And if you're buying a used car, there's a good chance it's riding on old tires. The age warning also applies to spare tires and even seemingly new tires that have never seen a mile 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 researches, analyzes and advocates 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 rated for higher-mileage longevity 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 advise customers to replace no later than the 10-year mark and encourage annual inspections after the fifth year.

But tire industry trade groups such as the U.S. Tire Manufacturers Association and Rubber Manufacturers Association argue that the five- to six-year guideline advocated by safety groups, and even automakers, is an arbitrary range unsupported by data. Not surprisingly, the tire industry resists legislation aimed at mandatory tire inspections after a certain age, countering that there are too many variables that determine tire aging, such as heat, storage, underinflation and typical driving conditions. To expand on these factors:

Heat: NHTSA research found that tires age more quickly in warmer climates. The NHTSA also found that environmental conditions, such as exposure to sunlight and coastal climates, can accelerate the aging process. People who live in coastal states and other areas with warm weather should bear this in mind when considering tire replacement.

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 still degrade with time. If a tire has been inflated and mounted on a wheel, it is considered to be "in service" even if it's never been used. And spares that are mounted underneath a vehicle — exposed to heat, dirt and weather — deserve extra attention.

Conditions of use: Has the tire been properly inflated? Underinflation causes more tire wear. Has it hit the curb too many times? Has it ever been repaired for a puncture? Tires on a car only driven on the weekends will age differently from those on a car that's driven daily on the highway. All are factors that contribute to the pace with which a tire ages.

Proper maintenance is the best thing you can do to ensure a long tire life. It's important to maintain proper air pressure, rotate tires regularly, and get routine inspections.

How to determine the age of a tire

The sidewall of a tire is covered in numbers and letters. They all mean something, but deciphering them can be a challenge. This Edmunds article about reading a tire's sidewall goes into greater detail. But to determine the age of a tire, you simply 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 1116 was made in the 11th week of 2016.

Tires made before 2000 have a three-digit number that is trickier to decode. The first two digits still indicate the week, but the third digit tells you the year in the decade that the tire 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. But this is all moot: If you see a three-digit DOT code today, that's a tire that belongs on a collectibles shelf or at the recycler, not on a car.

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

To make matters worse, you might not always find the full 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. Keep the visibility of the DOT number in mind the next time you are at a tire shop and the installer asks if you want the tires to be mounted with the raised lettering facing in.

After checking out a tire's birthdate, give the rubber a visual inspection. Some of the best advice on such an inspection comes from the British Tyre 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 said. It recommends replacing the tire immediately if such symptoms appear.

Don't buy used

Tires are expensive, especially after adding the costs of mounting and balancing, so it's tempting to consider used tires, especially if you're strapped for cash. Some small and major shops continue to offer used tires as an alternative, but when you buy a used tire, you have no idea how well it was maintained or the conditions in which it was used. The previous owner might have driven it with low pressure or repeatedly bumped it on curbs. It could have been patched for a nail. So while it's easy to determine a used tire's age, it's not so easy to determine its service life. Better to avoid it entirely.

Make sure you're getting a "fresh" tire

Just because a tire comes off the rack new doesn't mean it's actually a new tire. You don't need to search long to find a news or investigative story of a buyer who unknowingly purchased what they thought were new tires at a retail store only to learn that they were manufactured years earlier. In addition to having a shorter life on the road, an old tire sold as new may be past its warranty period.

Double-check the DOT codes on any new tires you buy. If you discover that they're really a few years old, you have the right to request newer ones.

Letting go

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

Although Kane has lobbied the NHTSA to enact regulations on tire aging, nothing is currently on the books. A 2014 NHTSA report lauded the current federal tire standard and stated the "agency will continue to monitor crash data to determine whether a tire aging requirement is warranted."

Since there's no official consensus from government or industry sources about when tires actually expire, we recommend following the general guidelines from automakers and tire manufacturers: Monitor closely and get annual inspections after five years and never wait more than 10 years before getting a new set. And don't forget the spare.