Dear Car Talk:
Do I really have to replace all four tires on an all-wheel-drive 2007 Ford Fusion when one tire is destroyed? -- Nathaniel
The problem is that all-wheel-drive cars have something called a center differential. A center differential is a box of gears that allows power to be transmitted to all four wheels, while at the same time allowing the wheels to turn at different speeds when they need to.
When do they need to? When you turn. Whenever you turn left or right, your inside wheels always turn slower (and travel less distance) than your outside wheels. If you don't believe me, steal one of your kids' Hot Wheels cars and turn it in a tight circle on the kitchen table.
Here's the problem: If you have one new tire that's larger than the other three, that new tire will always be turning more slowly, forcing the center differential to work. And the center differential is not designed to be in use all the time -- like when you're driving at 75 mph down the interstate.
If you think tires are expensive, Nathaniel, go out and price a differential. That said, there are a couple of situations in which you might not need four new tires.
If you bought the other three tires recently, you might get away with buying one new one. Every manufacturer has a limit to how much difference they allow in tread. So if the difference in the tires is 3-4/32nds of an inch, check with your manufacturer and see if that's allowable.
If not, and if your tires still have a lot of life on them, you can consider buying a shaved tire. No, that's not a new manscaping term you haven't heard of yet, Nathaniel. A shaved tire is a new tire that has its tread shaved down with a special machine to match the amount of wear on your other tires.
You might be able to find a tire shop locally that does it. If not, go to tirerack.com. They'll sell you the matching tire, shave it for you for $30 of so, and deliver it to you or to a local installer.
Even though it seems a bit wasteful, that might be the most cost effective solution of all, short of stealing all four tires from your neighbor's Fusion.