Good stuff, thanks.

BAck to my mpg question. Assuming my '99 Tacoma is not re-tuned when I put gasoline with ethanol in it, what happens to my mileage? I assume the car is smart enough to adjust the air mix and the timing, but then what? 10% ethanol drops the energy content by 3% relative to the straight hydrocarbon fuel. In practice does anyone know (via careful tests) what that does to mileage? This seems an important part of the arguments for or against using ethanol as motor fuel. I recognize that adding ehtanol likely means removing other high octane components from the original fuel so exact comparisons are of limited value, but...?

Is there some rough equation that relates energy content to mileage?

(mileage) = EC * a where EC is energy content and "a" is a proportionality constant
or
(mileage) = (EC - b)*a where b is energy lost to friction/heat

-d

In practice does anyone know (via careful tests) what that does to mileage?

There have been a number of independent tests done, and they show the expected decline in mpg. Most of these are on E85 though:

http://www.consumerreports.org/cro/cars/new-cars/news/2006/ethanol-10-06...

I have around here somewhere a University of Wisconsin-Milwaukee study from the 90's that looked at 5% ethanol blends, and concluded that they dropped fuel efficiency by around 2%. The American Coalition of Ethanol commissioned a study recently and came out with the conclusion that fuel efficiency generally drops, but by less than the BTUs would warrant. The ethanol guys like that study, but I don't suppose they would like a study funded by the American Petroleum institute that concluded just the opposite.