E-85 (or E85) is an abbreviation for a flex fuel that is a mixture of 85% ethanol and 15% gasoline or other hydrocarbon (HC). A flex fuel vehicle is one with an internal combustion engine that runs on more than one fuel, usually gasoline blended with ethanol or methanol. The term E-85 is sometimes a shorthand for ethanol, although technically it means the blended result of ethanol and gasoline.
To use E85, an engine must undergo modification to ensure that certain materials such as rubber are not exposed to the fuel, as the greater ethanol concentration will break them down. E-85 has a high octane rating, meaning that a more aggressive engine compression ratio can be used to generate more power. Since most E-85 vehicles are designed to run on both E-85 and gasoline, a compromise must be used in engine design in order to take into account the lower octane rating of gas. E-85 engines actually use more fuel to generate the same amount of power as a standard fuel motor. Ethanol is less expensive than gasoline by the gallon, somewhat making up for this deficiency.