Dear Tom and Ray:

I commute 240 miles a week, sometimes 480, to and from graduate school (I'm a doctoral student in political science). The trip is straight, flat and boring. My '91 Geo Metro convertible has 64,000 miles on it after two and a half years. I'm careful about getting it serviced, so the tires, hoses and belts look great and it runs well . . . but for how long? Can I expect this car to last until I pay for it? And just what is the ratio of highway-to-city driving for wear and tear? - Raena

TOM: Let's see, Raena. It's two and a half years old, and you want to know if it will last until you pay for it? Well, if you have a three-year loan, and you make a few accelerated payments, I think you can make it.

RAY: And the ratio of city-to-highway miles for wear and tear is 1.95628. My brother wrote HIS doctoral dissertation on that question.

TOM: Don't listen to him, Raena. He's got something against us PhDs (I think it's that we don't work). We have no idea what the actual ratio is. But we can tell you that highway driving is generally easier on the car. We explain this in detail in our pamphlet "Ten Ways You May Be Ruining Your Car Without Knowing It!" By the way, you can get your own copy by sending $3 and a stamped (52 cents), self-addressed, No. 10 envelope to Ruin No. 1, P.O. Box 6420, Riverton, NJ 08077-6420.

RAY: Basically, bumps, turns, potholes and starts and stops cause almost all of your car's parts to wear out faster (to date, we have found no effect on the carpeting in the trunk, but other than that, all parts seem to be affected). On the other hand, when you're driving on a smooth highway, you're simply not stressing those parts as much.

TOM: And on most cars, you're not stressing the engine, either. Because even though the car is going fast, the engine isn't turning any faster than when you drive around town at 35 miles an hour.

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RAY: Unfortunately, Raena, your car isn't like most cars. Your car has a three-cylinder engine; that's a little smaller than the motor in my brother's power knitting machine. So when you're doing 75 mph on the highway, your engine really is screaming. And this piddling, little engine just can't take going 75 miles an hour for hours and hours at a time.

TOM: So if that's how you drive, I'd go to the bank and pay off that loan tomorrow.

RAY: But if you're driving gently, obeying the speed limit, changing your oil regularly and doing your scheduled maintenance, you should get more miles out of your Geo Metro than other people get out of theirs. Good luck.

The Magliozzi brothers' radio show "Car Talk" can be heard each Saturday at 10 a.m. on KUER FM 90.1 If you have a question about cars, write to Click and Clack Talk Cars c/o King Features Syndicate, 235 East 45th Street, New York, N.Y. 10017.

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