Election Section

Click and Clack Talk Cars 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,

Tom and Ray Magliozzi and Doug Berman
Saturday October 27, 2001

Don’t switch tire size 


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. 




How to break into a car 


Dear Tom and Ray: 


My dear, departed daddy told me that if you broke in a new car engine at slow speeds, it would always be slow and sluggish. Is this true? What's the real skinny? Is there a preferred break-in protocol? – Susan 


TOM: Great question, Susan. But since we never speak ill of the departed, we can't answer it. 

RAY: Actually, I'm sure your daddy was right about many OTHER things and was a superior human being in all other regards. 

TOM: But his story about break-in is an old myth, Susan. And we don't know how it got started. Probably by some teen-age boy who got caught racing his dad's new car. 

RAY: It assumes that the car somehow "learns" to go slow when it's young, and then it never knows how to go at normal speeds later on. Kind of like my brother at work. 

TOM: But it's just not true. There IS a legitimate protocol for breaking in a new vehicle. It varies slightly from car to car, but the main purpose is to allow the piston rings to "seat," or conform to the exact shape of the cylinder walls so that they make a tight seal. And most experts agree that the best way to do this is to keep the engine rpm below 3,000 and to vary the engine speed (i.e., don't drive at one constant speed for a long time). 

RAY: And the break-in period generally lasts anywhere from 500 to 1,000 miles. Or until your check clears at the dealership, whichever comes first. 

TOM: If the piston rings don't seat correctly, your car might burn oil later on. And nobody wants that. 

RAY: So the speed at which you break the car in might have an effect on how much oil it burns. But it has absolutely no effect on how fast or slow the car goes. 

TOM: Last time we checked, that was mostly affected by the position of your foot on the gas pedal.  


Got a question about cars? Write to Click and Clack in care of this newspaper, or e-mail them by visiting the Car Talk section of cars.com on the World Wide Web. (c) 2001 by Tom and Ray Magliozzi and Doug Berman Distributed by King Features Syndicate, Inc.