[Drifter]

Again, you will burn more gallons of fuel to go the same distance but in doing so you should, in theory, use less oil.

1 gallon of pure gasoline - E0 - has ~115,000 btus
1 gallon of pure ethanol - E100 - has ~76,000 btus
1 gallon of E10 has ~111,000 btus (~3.5% less than gas)
1 gallon of E15 has ~109,000 btus (~5.2% less than gas)

So you should be getting ~5% (not 7% as I said earlier) worse mpg on E15 while, in theory, consuming ~15% less oil.

As Freebeard mentions, it does take considerable energy to produce ethanol (~35,000 btus worth of energy are needed to get ~76,000 btus worth of ethanol) so it isn't a "green" source of energy like the corn industry would like us to believe. But ethanol can be produced 100% domestically so your money stays in America rather than going to hostile leaders like Vladimir Putin, Saddam Hussein, Osama Bin Laden, etc.


Edit: And to be fair, it takes a lot of energy to produce 1 gallon of gasoline too - especially if the crude oil has to be transported half way around the globe.