E85 is actually for flex fuel vehicles--engines rated with higher compression. If using regular gasoline in engines suited for E85, it could cause the whole engine to explode.
The compression ratio of the engine determines the octane rating of fuel required. A flex fuel vehicle is one where the entire fuel delivery system (tank, pumps, injectors, lines/hoses, regulators, sensors) are all made of materials designed to resist the corrosive effects of E85, and most importantly, has a fuel content sensor that can determine the actual percentage of ethanol currently in your tank.
E10 is 10% ethanol 90% gasoline. It's mandated in about eight states. It creates less carbon emissions. This is because of the added oxygen in the fuel at the molecular level. Some states went with an additive called MTBE (methyl tertiary butyl ester, IIRC, but I ain't no rocket surgeon) which is an environmental disaster, IMHO-pollutes the water table much worse than ethanol. However by doing a little six month study on my own, I averaged about a 10% loss in MPG, therefor needing more, which in turn doesn't really lower the emissions I was putting out compared to 100% gasoline.
I would average 360 miles on one full tank of 100% gasoline and 335 on E10. Less energy content per unit volume for alcohol than gasoline, hence more volume used to obtain the same performance.
I go with pure gasoline which boosts milage and saves more than the $.10 that E10 charges less than pure.
Unless someone is looking to make an "environmental/social/political/economic statement" with their fuel choice, one needs to find the break even point where it becomes cheaper to use the ethanol blend.
*E-10 is NOT SAFE for carburetor engines without specific modifications, though safe for everything else.