Drivers often believe premium gasoline will help their cars run better, but, unless theirs is a sports car or luxury vehicle, it just isn't true. And with prices at the pump as high as they are, high-grade gasoline is one expense you may well do without.

According to Consumer Reports (search), most engines are designed for low-octane fuel (search). Octane is a measure of how gasoline performs in a one-cylinder test engine. The octane rating is based on a gasoline's resistance to engine knock, rattling sounds that occur when compressed fuel and air prematurely ignite. The higher the rating, the more resistant the fuel is. Regular fuel is usually 87, midgrade is 89, and premium is 92 or 93.

Premium gasoline (search) is usually necessary only for sports and luxury vehicles. The engines 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.

Putting premium gasoline in a car that doesn't need it won't harm the engine, but it won't benefit the vehicle, either. Use premium gasoline only if a car requires it.

To find out what type of fuel your car needs, first check the owner's manual. Some vehicles also note the ideal fuel type on or near the gas cap. Beyond that, you might check with your dealership's service department.

If you come across no mention of what fuel to use, start with regular-grade gasoline and listen carefully for pinging or knocking sounds in your engine. If there is repeated pinging or knocking, you may need higher-quality fuel — or a tune-up.