For economical car performance, choose the right gasoline

What's the right gasoline for your car? If you've never given it much thought, you could be wasting fuel and money by burning too costly a fuel for your car's performance requirements.

There is a right gas and a wrong gas for every engine.

Much depends on the engine's design, internal operating temperature, and even the region in which the car is being driven.

The right gas will give you the most economical performance at a worthwhile price. The wrong gas will cause detonation, a term synonymous with gas-guzzling, and abnormal combustion.

Detonation happens when gasoline, unable to withstand the heat and pressure of an engine's cylinders, ignites violently rather than burning smoothly after ignition.

This sudden ignition burst rattles the engine's cylinder head and makes a noise commonly heard inside the car and often described as a ``knock'' or ``ping.'' The noise is usually the result of burning gasoline with an octane rating too low for the engine.

The problem might also be eliminated with an engine tune-up.

It's when a tune-up fails to eliminate pinging that you should start trying fuel with higher-octane ratings until you arrive at one that silences the sound.

If you're now using an unleaded fuel with an octane rating of 87, increase it to 89, or the next-higher rating. Shell and Sunoco are both marketing sharply higher-octane unleaded fuel than the standard 87. Shell's product is 92 octane while Sunoco's is 931/2.

In the United States, federal law requires that gasoline-octane ratings be posted on the pumps. Refer to these numbers as you increase the octane of the gas you use, and don't get talked into using a premium gas by the attendant.

You might also consider changing brands of gasoline during your experimentation. The new-car syndrome

If you're unhappy about the gas mileage of your new car, there are factors to keep in mind before you start switching gasolines or taking other mechanical fuel-economy measures.

New cars have fuel-economy-estimate stickers on their windows, but in no way do these estimates specify the exact gas mileage the car will give.

Tests by the Environmental Protection Agency are run on dynamometers by skilled drivers using simulated stop-and-go driving techniques for relatively short mileage durations on the highway and in the city.

Thus testing does not take into account many of the extremes encountered in everyday driving, such as hot and cold weather, irregular road surfaces, hills and curves, strong head winds, poor driving habits, and the like.

Furthermore, a car's true mileage capability won't be apparent until the car is broken in.

The tight fit of new parts, including the stiff ride of new tires, creates friction, or drag, that prevents perfectly smooth performance. The break-in period normally takes several thousand miles. Figuring fuel consumption

To check fuel consumption and obtain a fairly accurate measure of your mile-per-gallon ratio, perform the following steps every six months or so:

Fill the fuel tank. When gasoline gets near the filler neck, pump gas slowly to prevent spillover and to allow air in the tank to escape. Record the odometer reading at the start of the test period.

Run the car at least 1,000 miles. Keep accurate records of how much fuel you add.

Drive for maximum fuel economy. Check your car's operating manual under ``nonmechanical considerations.'' Your car will use more gas in city traffic than on the highway at 55 m.p.h.

Compute the final mileage. The formula is simple. Subtract the starting mileage from the final mileage and divide the result by the gallons of fuel consumed. This gives you the fuel consumption in miles per gallon.

Mechanical factors

Get a tuneup. You can improve fuel economy by 5 percent or more just by having the car's idle speed adjusted to the recommended level. A change of spark plugs may make a similar difference.

Replace the air filter. Your car's engine consumes, by weight, about 14 or 15 times as much air as it does gasoline. A dirt-clogged air filter lowers this ratio, increasing emissions and robbing you of fuel.

Check the front-end alignment. Clean oil of the right grade can do a better job of putting a slippery film between the moving parts of a car's engine and thus keep them from rubbing together and wearing. The engine has less resistance to overcome. The resulting smoother performance can shave 3 cents off the price of a gallon of gasoline.

Consider changing the tires. If your car presently has bias-ply tires, you could probably improve the fuel economy by switching to radials. Most consumer-group and government studies indicate that radial tires typically yield fuel economy 4 to 6 percent higher than bias-ply tires, because the radials offer less rolling resistance. It also helps to keep the tires properly inflated to avoid drag. Nonmechanical factors

Some of the best gas-saving steps are the ones you can do yourself. Good driving habits will add up to gas savings that won't get blown out the exhaust pipe:

Avoid jack-rabbit starts. They burn twice as much fuel as normal getaways. Slow starts, getting into high gear as quickly as possible, and moderate, consistent driving habits will improve your fuel-economy record significantly.

Avoid stop-and-go passing. Avoid tailgating or running up close to the car in front of you, slamming on the brakes, and then hitting the accelerator pedal to pass. First, wait for a clear road ahead. Then start to pass well to the rear of the car in front of you so you can swing out smoothly and negotiate the pass without heavy braking or accelerating.

Stick to the speed limit. Most cars get about 20 percent more miles per gallon at a steady 55 m.p.h. than at 70. That's a saving of about 4 gallons in every 20. Besides, the police won't bother you if you obey the speed signs.

Keep from braking unnecessarily. If you can, keep rolling and do not stop. This means trying to time traffic lights, keeping in mind the traffic around you on the roadway.

If a light in the distance is red, coast up and brake slowly. If the light turns green before your car reaches the intersection, apply steady pressure on the accelerator pedal. Stay off the brake pedal if you can.

Don't race the engine after cold-engine start-ups. Your car always consumes a few ounces of gasoline during start-up and about half the starting amount for each minute of idling. Driving slowly for the first couple of miles is a much better way to warm up the car.

If you pump the accelerator pedal repeatedly before start-up, you might be wasting as much as an extra gallon of gasoline a week. Depressing the gas pedal just once before starting is all that most cars require.

Short trips are one of the biggest gas gulpers, too. Because the engine usually can't warm up to its full efficiency, a short trip can require up to 70 percent more gasoline per mile than a longer trip.

Don't overuse the accessories. This applies especially in the warm months when air conditioners are widely used. When you flick on the air conditioner while traveling down the highway, your car's gas mileage immediately drops by about 10 percent.

When you do the same in heavy stop-and-go traffic, gas mileage can fall by as much as 20 percent.

Other accessories, such as lights, heater, defroster, and radio, consume extra fuel as well. But the biggest energy user, excluding the engine itself, is the air conditioner -- by far.