The octane level of gasoline refers to its ability to prevent engine knock. This occurs when the mixture of air and fuel in a cylinder ignites prematurely. The Federal Trade Commission recommends using the octane level of gasoline recommended in your owner's manual.
Most cars use "regular" gasoline, which is usually 87 octane. Regular gasoline is some areas of the U.S. is 85 octane. The yellow sticker at the gas pump states the octane level of the gasoline. You should not use 85-octane gasoline if your owner's manual recommends a higher octane level.
Lower air pressure at high altitudes lowers the pressure in your car's cylinders; this might allow you to use a lower octane fuel. Eighty-five octane fuel at high altitudes is equivalent to 87 octane at sea level, according to Car Talk.
You are using an appropriate level of octane gasoline if your engine does not make rattling or pinging noises. Switch to a higher octane gasoline if the engine rattles or pings.
Using a higher octane gasoline than recommended by your owner's manual won't improve gas mileage, make your engine cleaner or improve engine performance, says the FTC.
