Dear Car Talk:

We have two cars that are each driven about 4,000 miles a year. Since it will take 10 years or so to put 40,000 miles on the tires, should I consider replacing them after a certain number of years, instead of miles? If so, how many years do you recommend for replacement?

-- Bob

Well, the American tire industry recommends that tires be replaced every six years, even if the tread is not worn down. Of course, the American Couch Association also recommends that you replace your sofa cushions every three months due to flatulence. And I think both of those industries probably are erring on the side of caution. And increased sales.

Tires do dry out and degrade over time, due to exposure to sun and ozone in the air. I'm sure you've seen old tires that are covered in small cracks on the sidewalls. Those should be replaced.

They also can degrade in and around the tread, where most people don't look. So that needs to be checked, too.

So my advice would be, starting at year six, have a mechanic you trust (and not necessarily a tire salesman) take a good look at your tires. You need a knowledgeable, non-interested party to do a visual inspection and tell you whether the tires are still good.

If your cars sit outside all the time and you live in a hot-weather climate, and you bought cheap tires to begin with, you may need tires after six years. If they're garaged, or they're particularly good tires, you could get seven, eight or even more years out of them while still driving safely.

And you want to factor in your driving habits, too. If 95 percent of your driving is from your house in a golf community to the clubhouse for lunch and back, the risk of catastrophic failure is low, because heat from high-speed driving is what tends to make tires fail suddenly.

Whereas, if you do a lot of highway driving, you'll want to err on the side of caution and make sure your tires are not near the end of their lives.

By the way, if you're not sure how old your tires are, you can check the sidewall. You'll find a number there that says something like "2214." That means your tires were manufactured during the 22nd week of 2014.

And if they're not 6 years old yet, at least you'll know when to throw them their next birthday party.