Gasoline BTU= 125,000 ethanol= 76,500.
The more alcohol the lower the mileage.
To put it another way, A tank of gasoline (in a particular car) will go 400 miles. That same tank full of alcohol would go 240 miles.............
BTU's are a great way to measure the ability of a given fuel to boil water - they're less accurate as a way to measure the amount of power available to drive a piston.
While an engine optimized for gasoline will suffer poorer mileage when fueled with ethanol, an engine optimized for ethanol (generally a higher compression ratio and advanced spark) suffers lass of a mileage drop and has more power available.