[benji1997]

The differences in octane number originate in chemical mixtures created at the refinery, and different grades of gasoline are kept separate throughout the distribution process. The right grade to buy is the one your particular car was engineered for; you need to use only a grade high enough to prevent knock. ''Putting premium in a car designed to run on lower octane is a waste of money,'' said Dennis Simanaitis, the engineering editor of Road and Track magazine.
Using regular in a car designed for premium, however, is a mistake. ''If a car is rated to run on premium, it will still run on 87 or 89, but the engine computer will dial back its performance and you won't get optimum performance or gas mileage,'' Mr. Simanaitis said. In that kind of car, ''using premium fuel will eliminate the knock,'' he added, ''and the engine computer will deliver full performance.''
So which grade of gasoline is right for your car? Look in your owner's manual.