"There are all sorts of units of measure to quantify energy content; you can measure it in British Thermal Units, joules or even calories, just like you do food. A standard gallon of 93-octane gas contains about 114,500 BTU of energy, which is around 450 calories, or the energy equivalent of a double cheeseburger. A gallon of diesel runs about 129,500 BTU, and pure ethanol measures out to about 76,100 BTU. If you're wondering why these fuels -- which are all liquid hydrocarbons -- have different energy contents, you need only look at their relative densities. Diesel is the heaviest, followed by gasoline and then ethanol.
Where pure ethanol and E85 ethanol -- 85 percent ethanol, 81,800 BTU -- triumph is in octane rating. Ethanol has an octane rating of about 108.6, which is only 0.10 octane lower than the methanol once commonly used in racing circles. But that's pure ethanol, not E85. While mixing 85 percent 108-octane ethanol with 15 percent 93 octane would yield something in the 102 octane range, the fact is that most manufacturers mix it with 84 octane or lower. That yields a final fuel rating of between 96 and 98 octane."
The BTUs are less, which is why you need more e85 when compared to gas.