Ethanol has a lower density than gasoline. So, although it has a similar mass-specific energy content, the volume-specific energy content is lower. Ethanol has a greater resistance to ignition than gasoline. But, since consumers buy gasoline in grades based on the octane rating (usually (RON+MON)/2), you don't see any octane benefit in your tank and do get reduced energy content.
It's a scientific fact based upon the free energy content of the same amount of gasoline versus the same amount of ethanol. Ethanol just doesn't have as much energy as regular gasoline. If you've observed the opposite effect, you're experiencing some kind of confirmation anecdote. I can almost promise something else is at work, such as less wind pushing on the nose of your car, different temperature, tire pressure, hills, something.
E85 is really bad, and will cost you 20-25% of your MPG compared to burning the equivalent amount of gasoline.
EDIT: The U.S. government publishes figures more conservative than what I just said, based on the link above me that I just read. They cite NREL/TP-540-43543[1].
It's a scientific fact based upon the free energy content of the same amount of gasoline versus the same amount of ethanol. Ethanol just doesn't have as much energy as regular gasoline.
This by itself is meaningless. Diesel also has less energy than regular gasoline, yet diesel vehicles can generally beat equivalent gasoline vehicles in terms of mpg. There are more factors than simply energy content!
Whoops! Well, anyway, I think my point still- energy content is not the only factor- as diesel can get much more than 12% improved fuel economy. Diesel runs at much higher compression ratios, which is a good part of the increased economy. There's other things too, like the lack of a throttle plate which reduces pumping losses.
So to compares prices per gallon you need to derate the E15 (and the E10).
Does anyone have the numbers? Like 1 gallon of E10 is equal to .9 gallon of E0, etc?