[TFMcGuire]

30% is a fair rule of thumb, although as you say, a couple of energy transformations occur.

In modern low compresssion engines, the octane benefit does not offset the reduced chemical energy of the molecule.

Ethanol does cause a significant reduction in vehicle mileage with family cars---even those able to run on E-85.

[Mr. Lucky]

I agree. But in a high compression engine, or a low compression engine with a supercharger, the fuel economy advantages of gasoline can disappear. The reason I say "can" disappear is because a properly tuned alcohol engine will generate so much more power than an 87 octane gasoline engine of the same displacement. That increase in power can manifest itself in fuel economy if the driver is able to "keep his foot out of the carburetor" as we used to say. Even at high prices, however, most of us can't resist applying the extra power to increase performance rather than fuel economy.