Do all of the tires need to be replaced on this Ford Fusion?

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

RAY: You might.

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.

When in doubt, drivers steer toward the shoulder

Dear Car Talk:

All my life I’ve noticed that some people veer abruptly to the right onto the shoulder when braking suddenly.

I’ve always wondered if these people are doing this consciously for some reason, or if it’s unconscious, or maybe their brakes are pulling to the right.

If they are doing this intentionally, why? – James

RAY: I think it’s to avoid bashing into the car in front of them, James. If you’re driving along and the car in front of your stops suddenly, what do you do?

OK, what do you do after you’ve run through all your four-letter words?

Well, you slam on your brakes to try to stop before you plow into the car in front of you, right? And if there’s any question about whether you’ll be able to stop in time, you steer your car to the right, onto the shoulder.

You could steer your car to the left, but you’d be driving into oncoming traffic. And Darwinism has already removed most of the “brake and steer left” genes from the human gene pool.

So the shoulder is the obvious place to point the car. If you’re lucky enough to be able to stop in time, no harm done. And if you’re not able to stop in time, you’ll pass the stopped car on the right instead of testing your car’s airbag (Hey, good news, Hon, our airbag works!).

I suppose it’s possible that in some cases, the crown of the road (which slopes down to the right) may direct some cars to the right in an emergency stop, but cars are built to go straight when you hit the brakes.

I’m guessing it’s human ingenuity in action here, James.

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