Fuel mileage question

Ethanol

Ethanol has a a much lower heat content than gasoline. What this means is you will get poorer fuel economy on ethanol than with gasoline. In the case of E85, the heat content (BTUs per gallon) is something like 40% less than gasoline - hence you'll get substantially lower fuel economy on E85.

Ethanol does have advantages however - a higher resistance to detonation than regular gasoline. E85 has an equivalent octane rating of 105, vs 87 for regular gasoline. Also, because ethanol has oxygen in it (formula for ethanol is C2 H5 OH) it is possible to get more power out of an engine using ethanol vs the same engine with gasoline - however to accomplish that requires modifications so that the engine can burn much more ethanol by volume than gasoline.

Bottom line is burning E85 in an engine made for gasoline will result in very poor operation. Burning E10 will result in slightly lower fuel economy, but probably acceptable engine performance.