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.
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!