How much corn is needed to make one gallon of ethanol?
About 26 pounds.
Estimates seem to be in the range of 2.5 - 2.8 gallons / bushel (54 lbs), which sounds better than I would have guessed - ethanol weighs 6.6 lbs / gallon, suggesting that they are getting around 1/3 of the corn’s weight in ethanol.
It doesn’t have to be just corn. Loran Balvanz, president & CEO of US Manufacturing, has proposed using industrial waste from companies like Perdue Farms and Weyerhauser.
I have some related questions. Working backwards:
How much energy is expended to distil corn into 1 gallon of methanol alcohol?
How much gasoline or diesel fuel is burned to deliver this amount of corn to the distillery?
How much diesel fuel is burned (by the tractor) to harvest this amount of corn?
How much diesel fuel is burned (by the tractor) to sow this amount of corn seed?
How much gasoline or diesel fuel is burned to deliver t this amount of corn seed to the farmer?
How much gasoline or diesel fuel is burned to harvest this amount of corn seed?
You get the drift. 
So someone… please add up all the energy that was expended to:
a) Distil corn into 1 gallon of alcohol.
b) Deliver enough corn to distillery to make 1 gallon of alcohol.
c) Pull enough corn to make 1 gallon of alcohol.
d) Sow enough seed to make 1 gallon of alcohol.
e) Deliver enough seed to make 1 gallon of alcohol.
f) Make enough seed to make 1 gallon of alcohol.
After you did that, compare the amount of energy in 1 gallon of alcohol with the amount of energy it took to produce it. Do you come out ahead? In other words, is there a net energy gain?