In an engine optimized for ethanol, the difference in BTU content doesn't strictly correspond to difference in mileage. An engine tuned for ethanol can have a substantially higher compression ratio, the spark can be advanced, the cooling system can be modifed and the like.
True; still doesn't change the ratio between the two for purposes of arithmetic.
What we must be careful to remember is that currently the focus is on alternative fuels and ramp up times.