Originally Posted by MillerTime
Because of the higher octane you get less mpg. The research I saw said about 5-15% decrease in mileage.
That is not entirely true.
E85 gets worse mileage than premium gasoline, as well as midgrade and regular gasoline. Octane ratings donít have a direct impact on fuel economy, and premium isnít necessarily a better fuel for your car if it doesnít require the high-grade fuel. It is the energy density that impacts fuel economy.
E85, which is 85 percent ethanol and 15 percent regular gasoline, has a lower energy density than regular gasoline; a car has to burn more E85 to go the same distance as a car with regular gasoline, which means worse fuel economy. This is akin to how diesel has more energy density than gas and as such better mileage than gas.
E85 does however allow a properly designed engine produce some big power. For example anyone remember Ford's Super Stallion.
Ford's 590-Hp Super Stallion - Exclusive Test - American Car - Motor Trend Magazine