Making ethanol uses up non-corn (ordinary) energy. If it uses more more than is in the ethanol, the "net energy balance" of ethanol is negative. But it would still save gasoline because a lot of the energy used to make ethanol comes from coal. In effect we would be turning coal into gasoline. But burning coal makes more CO₂ than burning gasoline, so this would increase greenhouse gas emissions.

Contrary to ethanol critics like Pimentel and Patzek, the best studies indicate ethanol's net energy balance is positive—but not by a lot. Most studies find it takes 74% to 80% as much non-corn energy to make ethanol as is in the ethanol made. The diagrams below assumes 75%, which is optimistic compared with the latest report by the National Academy of Sciences. It also doesn't count nuclear or hydro inputs, only fossil energy inputs, and it overstates, by a little, the energy needed to make gasoline.

Just remember, a positive net energy balance does not tell us that greenhouse gases are reduced, how much energy imports are reduced, or what all this costs.

Using ethanol save fossil two ways: 

(1) One quarter of the energy in ethanol comes from corn, nuclear and hydro (mostly corn).

(2) Replacing a gallon of gasoline replaces about one quarter more energy than in the gallon, becasue fossil used to make gasoline.

The energy in a gallon of gasoline is called 1 GGE of energy.

Making and using 4 GGE of ethanol requires 3 GGE of fossil fuel, but saves the (almost) 5 GGEs of fossil fuel needed for the 4 gallons of gasoline replaced.

This means, 4 GGE of ethanol saves 2 GGE of fossil fuel.

But ethanol has only 2/3 the energy of gasoline, so 4 GGEs of ethanol is 6 gallons of ethanol. It takes 6 gallons of ethanol to save 2 GGE, the energy in 2 gallons of gasoline.