Scientific American, January 2007, p.46:
The first problem is that a standard barrel (42 gallons) of ethanol is worth about 28 gallons of gasoline because it contains 80,000 British thermal units (Btu) of energy, versus about 119,000 for unleaded regular If you fill your tank with E85, you will run dry about 33% sooner. Even if a gallon of ethanol were cheaper at the pump, drivers would have to buy many more gallons to go the same distance.
...
Producing a gallon of ethanol, with its 80,000 Btu of energy currently requires about 36,000 Btu of natural gas.