Dear Tom and Ray:

My son, who is a certified mechanic and sells lots of tires, tells me that if I go to a higher-profile tire on my '98 Chevy Cavalier for a softer ride, terrible things will happen: The computer will sizzle, gas mileage will drop to 4.3 miles per gallon and I will roll the car on the first lane change. He says he has been told this in his certification classes (he has stripes down his sleeve like a sergeant major). On the other hand, I have a couple of college degrees and a lot of experience, so I don't believe it. What do you clowns believe? -- Richard

TOM: Gee, Richard, I'm surprised your son didn't mention the rapid hair loss and the rash on your butt that will last a month.

RAY: The kid does have a flair for exaggeration, Richard, but the truth is, he's right. He's right to discourage you from switching to a non-manufacturer-recommended tire size.

TOM: Here's the story. When a car is designed, it's tested for handling and emergency-control characteristics with a certain size and type of tire. If you change that, you, by definition, alter the handling characteristics of the car. Enough to fry the computer? No. Enough to seriously alter your gas mileage? No. Enough to flip the car in a lane change? Probably not.

RAY: But the problem is, nobody knows exactly what the new handling characteristics of the car will be, because the car has never been tested using higher-profile tires.

TOM: Now, if you make a modest change in tire size in one direction or the other, chances are the car will still be safe. In fact, you probably won't even notice any difference; most cars probably have a reasonable safety margin built in. But if you're dealing with a sport utility vehicle or something else that already handles peculiarly, then changing the type or size of the tires can be very dangerous. And that's why your son is taught to recommend against it.

RAY: And we're with him, Richard.