This is an important question in light of the escalating prices at the fuel pump and the temptation for many to go to the cheaper regular fuel in spite of the fact that their owner's manual recommends premium.
HERE ARE THE FACTS:
In most of the U.S., regular gas has an octane rating of 87, midgrade gas is 89, and premium is 91 or 92.
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.
Knocking is caused by uneven combustion in the cylinder. You need a gasoline with a sufficient octane rating to
prevent this knocking in your car's engine.
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 and you should use only the recommended grade of gas.
On the other hand, using high-octane gas in a car designed for regular accomplishes little except more rapid combustion of your money. Engines designed for regular fuel don't improve on premium and sometimes run worse.