Dear Tom and Ray:
I just bought a new Jeep, and the owner's manual says that all tires, including the spare, should be replaced after six years, regardless of condition or usage, to avoid a sudden failure during use. I don't remember seeing this recommendation before. The spare in my last Jeep is now more than 10 years old. Should I replace it? What is the reasoning behind this recommendation? — Eric
Tom: What's the reasoning? Well, the Goodyear pension plan is seriously underfunded.
Ray: Actually, it's about the deterioration of the rubber, Eric. If you take a rubber band and toss it in your kitchen drawer, when you go to stretch it a year later, what happens? It's all dried out, and it breaks.
Tom: There's a similar, though much slower, process happening with your tires. Over time, the ozone in the air degrades rubber. Just from being in Earth's atmosphere, tires dry out, crack and, eventually, fail to hold air.
Ray: So how'd they come up with the six-year time frame? Well, it's somewhat arbitrary. They looked at a number of factors: the rate at which rubber decays, how the average person cares for his or her tires, the real-life data on tire failure and the tire sales numbers for Q4. They put it all together, and they came up with a guess of six years.
Tom: So, it's a guess. Your tires may last longer or may fail sooner. But it's a reasonable guess that errs on the side of safety. And in reality, most tires have their tread used up in less than six years anyway. So it's only an issue for people who don't drive much, and for spare tires that don't get rotated into the mix.
Ray: You may have a little more leeway with your spare, since you're not actually driving on it every day. But in an emergency, if you were forced to use it, you'd have to drive slowly and carefully, and then replace it as soon as possible. If it were me, I'd replace a spare that's 10 years old.
Tom: And speaking of replacement tires, you now have one more thing to think about at the tire store. Like bread and milk, you now have to make sure your tires are "fresh."
Ray: Right. If tire manufacturers are telling us that tires have a six-year shelf life, regardless of use, then you don't want to buy tires that have already wasted a year of their useful life stacked up in a retailer's showroom or an overheated storage trailer.
Tom: How do you know when your tires were made? It's on the tire. One of the numbers printed on the sidewall is a four-digit number, like 1711. That means the tire was made in the 17th week of 2011. Now, wouldn't it be easier if they took a lesson from milk and printed an "expiration date"?
(Car Talk is a nationally syndicated column by automotive experts (and brothers) Tom and Ray Magliozzi. Write to them at the Car Talk website.)