NEW YORK – Many people believe that buying premium gasoline will help their cars run better, but unless you own a sports car or luxury vehicle, it really won’t make a difference. With prices at the pump poised to rise yet again, buying high-grade gas is one expense you could well do without.

You should pay for premium only if your car requires the fuel, advised Mark A. Baxter, director of the Maguire Energy Institute at Southern Methodist University in Texas. “If you think it’s going to run faster or you’re going to get better mileage, don’t waste your money,” he said.

According to Consumer Reports, most engines are designed for low-octane fuel. Octane ratings, a measure of how gas performs in a one-cylinder test engine, are based on a gasoline’s resistance to engine knock – rattling sounds that occur when compressed fuel and air prematurely ignite. Higher ratings correspond with more resistant formulations: Regular fuel is usually 87, midgrade is 89 and premium is 92 or 93.

Premium gas is usually called for by makers of sports cars and luxury vehicles, which have engines that are more prone to knocking and can be damaged with repeated use of low-octane fuel. Some have special features that allow them to run on regular gasoline, but at a lower level of performance.

While pumping premium into a car that doesn’t need it won’t harm the engine, it won’t benefit the vehicle either. “It just causes damage to your billfold,” added Baxter.

To find out what type of fuel your car needs, first check the owner’s manual. Some vehicles also have a label noting the ideal fuel type on or near the gas cap. You might also check with your auto dealer’s service department.

If you can’t find out what fuel to use, Baxter recommends trying the regular gasoline and then listening carefully for pinging or knocking sounds from the engine. If there is repeated knocking or pinging, you may need better-quality fuel – or a tune-up.

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AP-NY-08-17-04 1732EDT