A few years ago Vermont had still not begun adding ethanol to its gasoline and I, being a New York state resident, was able to use both “10% ethanol added” and “no ethanol gasoline” from time to time. I recall getting about 10% less mileage with the New York ethanol added gasoline, though my ability to measure was admittedly crude. In any case, at no time did I get greater gas mileage with the ethanol added product. My impression is that adding water to gasoline would produce similar mileage results to ethanol-adulterated gasoline.
Ethanol simply doesn’t have the BTU content of gasoline;I’ve read that German WW2 aircraft performance was limited by use of alcohol fuels.You simply need more volume of alcohol than gasoline.