Dear Tom and Ray:

My wife, Gert, and I read your column every week and enjoy it. Please clear something up for us about gasoline octane. There is 87, 89 and 93 at our local gas station. Which one is best for our cars? We have a '93 Chevy Lumina and an '86 Mercury Grand Marquis. I am under the impression that the higher-octane stuff is better because it helps clean the engine. My wife says no. What do you say? -- Bobby

Tom: Bobby, you should always listen to your wife when it comes to cars! Haven't you learned that from reading our column every week?

Ray: Gert is right in this case. Ten or 15 years ago, when fuel injection was still relatively rare, oil companies advertised that they added special detergents to the higher-octane gasolines. The reason, they said, was that these detergents helped clean fuel injectors.

Tom: But now all cars are fuel-injected, and oil companies say they add these detergents to all grades of gasoline. So, as far as we can tell, there's no longer any reason to buy any more octane than you need.

Ray: How much do you need? In your case, 87 is fine, Bobby. You've got two run-of-the-mill cars, which call for run-of-the-mill gasoline. The higher-octane fuels are designed for "high compression" engines, which need a higher ignition temperature to keep from pinging.

Tom: If you're in doubt about how much octane you need, check your car's owner's manual. It'll tell you what octane you need to keep the car from pinging. Any more just creates more pollution and wastes your money.

Changing your oil regularly is the cheapest insurance you can buy for your car, but how often should you change it? Find out by ordering Tom and Ray's pamphlet "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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