http://www.straightdope.com/columns/...nd-regular-gas
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In most of the U.S., regular gas has an octane rating of 87, midgrade gas is 89, and premium is 91 or 92. (Octane ratings are lower in the mountain west due to the effects of thin air on internal combustion.) Contrary to widespread belief, the octane rating doesn't indicate how much power the fuel delivers; all grades of gasoline contain roughly the same amount of heat energy. Rather, a higher octane rating means the fuel is less likely to cause your engine to knock or ping. Knock, also known as detonation, occurs when part of the fuel-air mixture in one or more of your car's cylinders ignites spontaneously due to compression, independent of the combustion initiated by the spark plug. (The ideal gas law tells us that a gas heats up when compressed.) Instead of a controlled burn, you get what amounts to an explosion--not a good thing for your engine. To avoid this, high-octane gas is formulated to burn slower than regular, making it less likely to ignite without benefit of spark.
The majority of cars are designed to run on regular gas, and that's what the manuals tell the owners to use. Higher-performance cars often require midgrade or premium gas because their engines are designed for higher compression (higher compression = more power), and regular gas may cause knock. If your car needs high-octane gas, the manual will say so.
Using high-octane gas in a car designed for regular accomplishes little except more rapid combustion of your money. Some refuse to believe this, claiming, for example, that premium gives the family Toyota better mileage or more power. These people are in dreamland.
Others say premium is purer or contains detergents that will cleanse your engine of uncouth deposits. Likewise misguided thinking--government regulations require detergents in all grades of gasoline. (BP Amoco, I notice, asserts that its premium gasoline contains more detergents than legally required; if you think that's worth 20 extra cents a gallon, be my guest.) Some automotive types claim that using premium in a car designed for regular will make the engine dirtier--something about deposits on the back side of the intake valves. I've also heard that slower-burning high-octane gas produces less power when used in ordinary cars. Believe what you like; the point is, don't assume "premium" means "better."