The grades of gasoline offered at retail stations are based on octane rating. The octane rating is clearly posted on each dispenser using a yellow label with black lettering similar to the example below. The octane rating is a measure of the anti-knock characteristics of the fuel. Knock occurs when the air-fuel mixture in the cylinder spontaneously ignites before the spark plug fires. Knock can be recognized by a pinging or clicking sound in the engine particularly when climbing a steep hill or pulling a heavy load.
Refiners produce gasolines with a range of octane ratings.
Most cars will perform acceptably on 87 octane gasoline. Fuels with octane ratings of 89 to 94 resist the tendency to knock, but do not necessarily result in more power or better overall performance. Consult your owner's manual for information on choosing the correct fuel for the model and engine configuration in your car or truck. According to the FTC, buying gasoline with a higher rating may not improve engine performance or increase mileage to justify the increased cost. Don't be fooled by the product names like premium, platinum, gold, extra, special, etc.