What is octane and why is it important?
Octane measures a fuel's resistance to engine knock. The right level of octane prevents engine knock and ensures optimum performance. The most common levels of octane are 87 (regular), 89 (mid-grade) and 93 (premium).

What is engine knock?
Engine knock is uncontrolled combustion associated with using gasoline with too little octane. The knocking or pinging sound may be more noticeable when accelerating or climbing hills.

What if I use gasoline that doesn't have enough octane?
If you hear knocking, it could, over time, damage pistons and other engine parts. If your car has a knock sensor, it will compensate by slowing spark timing. Your engine will be protected, but possibly at a slight and sometimes noticeable loss in power and acceleration. Roughly half of today's vehicles are equipped with knock sensors.

What if I use premium but don't really need it?
Most cars give optimum performance on regular or mid-grade gasoline. If you're buying premium and your car's not running any better than it does with a lower-octane gasoline, you're probably wasting money. However, some cars may operate better on premium because of additives. Additives, which are found in all gasolines, keep engines clean and make them run more efficiently. Some brands have more effective additives and some use bigger doses in their premium grades.

Do driving habits affect the amount of octane my car needs?
Yes. Driving habits, a vehicle's mileage, and climate and geography can affect how gasoline performs in your car. Octane requirements tend to increase with mileage, at least through the first 15,000 to 20,000 miles. And a car hauling heavy loads over hills requires more octane than the same car driving on level roads. On the other hand, many older vehicles need less octane at higher altitudes, such as the Rocky Mountains.

How do I know which grade of gasoline to buy?
The octane level recommended by your vehicle's manufacturer is a good starting point. But, price, driving habits and personal preference also are important. And so are individual vehicle characteristics. For example, research shows that cars the same age with identical engines have different octane requirements, probably due to manufacturing tolerances. The best advice is: try different gasolines, observe how they perform in your car, and pick the one that meets your needs.

Do motorists use too much premium?
API's study compares U.S. Department of Energy estimates of gasoline sales by grade and the Coordinating Research Council's data on vehicle octane requirements by model year. CRC is a nonprofit organization that has evaluated U.S. octane requirements, annually, for almost 50 years. Its data show that, as of 1994, 19 percent of vehicles on the road required premium gasoline. DOE figures show that 20 percent of gasoline sold in 1994 was premium. That means Americans buy about the right amount of premium gasoline.