Leah, here's the problem ( and it's not an easy one to answer ).

If you put gasoline or ethanol in your fireplace and burn both 100%, gasoline has way more energy expressed in BTU.

But gasoline is not burned 100% in your car, some goes out the tailpipe, and only a small percentage ( maybe 40% at best ) is used to move the car. The rest is lost as heat.

Gasoline has 115,000 to 125,000 btu per gallon.
100% ethanol has 68,000 to 76,000 btu per gallon.

A BTU is a British Thermal Unit. A BTU is the amount of heat (energy) required to raise the temperature of one pound of water one degree Fahrenheit. But the #1 use of gasoline is not to boil water, now is it?!

High BTU content in a gasoline does not necessarily mean the engine will make the most horsepower on that fuel.
Many car enthusiasts want the gasoline with the highest BTU content per gallon. Unfortunately, BTU content is of little value if some of the gasoline is still burning when the exhaust valve opens and all of that energy escapes out the exhaust as heat and unburned
hydrocarbons. Most gasoline engines do not have time to completely burn the gas in the combustion chamber.

A hypothetical example:
A gasoline car may use 115,000 BTU gas, but only burn 80% if it before the exhaust valve opens. A properly tuned ethanol car running E85 with 83,000 BTU ( the 15% gas boosts the BTU, and the ethanol boosts the Octane to 105 ) may burn 95% of the fuel before the exhaust valve opens.

So take your pick:
Gasoline: 80% of 115,000btu = 92,000 btu per gallon actually utilized.
E85: 95% of 83,000btu = 78,850 btu per gallon actually utilized.

The higher octane of E85 means you can build a higher compression engine and still avoid backfire and knock, and get more horsepower per BTU.

Take the above, and gasoline powered cars, and specifically built ethanol powered cars ( Indy Racers ) have a level playing field.

In testing using a 740 horsepower 358 cubic inch racing engine* with 14:1 compression ratio, the engine made 5 more horsepower on a 100 octane unleaded RFG (like 20% ethanol) than on 110 octane leaded racing gasoline, because the RFG contains an oxygenate
( more oxygen ) that allows for more complete combustion.

NOW consider it costs about $1.05 to $1.15 to make one gallon of ethanol, and ethanol usually sells wholesale for $2 or less. Add that to that fact that it is clean burning, and is renewable. Now, I feel, ethanol has the advantage, and is the best choice in areas where you have a choice!

-John

*Source: Rockett Brand Racing Fuel