Dear Tom and Ray:

I heard a story on my local NPR station about a place in Houston where car owners can have the air in their tires replaced with nitrogen at a cost of $100. This service is advertised as a fuel economy enhancement because nitrogen diffuses through the tire at a slower rate than air, and so the tires stay at the proper pressure longer. Is this a scam, or what? -- Mark

Ray: Yes. And a very clever one at that. In fact, my brother's on the phone right now seeing if we can get the local nitrogen-transfusion franchise around here.

Tom: Their claim that nitrogen keeps your tire pressure more constant is actually correct. But their reasoning is wacko. It has nothing to do with diffusion of air through the rubber.

Ray: The reason nitrogen pressure stays more constant than air pressure is because air contains water vapor, and so it expands less predictably than nitrogen. There's a different percentage of water vapor in the air on any given day (a k a the humidity), so you never know exactly how much tire expansion you'll get from the water vapor.

Tom: It's not enough expansion for you or me to notice, but some race-car drivers notice it. Formula One (i.e., Indy 500) racers use nitrogen in their tires because when you're traveling around an oval track at 200 mph, you want your tire pressure to be entirely predictable. They even "stagger" the tire pressure on those cars, making the outside tires a little fuller than the inside tires to keep the car turning inward. And at those speeds, an eighth of an inch in tire height can make a huge difference.

Ray: But for you and me (and everyone else reading this column today), Mark, it makes absolutely no difference at all. And it's simply not worth it (unless, of course, you can steal the nitrogen from work!).

Tom: Seriously, you can keep your tire pressure constant enough for street and highway driving by simply checking it periodically.

* * *

It's amazing how many bad driving habits and wacko theories have been passed down from generation to generation! Tom and Ray set the record straight in their booklet "Ten Ways You May Be Ruining Your Car Without Even Knowing It!" To order, send (check or money order) to Ruin, P.O. Box 536475, Orlando, FL 32853-6475. You can also order online.

© 1997 by Tom and Ray Magliozzi and Doug Berman Distributed by King Features Syndicate, Inc.

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