From what I've read the fuel economy hit is about 3% maybe as much as 4%. Ford and Toyota claim 3%. I don't see any reason to believe otherwise-except when something has gone very wrong. That's 1 MPG on a 30 MPG vehicle, and .3 MPG on a 10 MPG vehicle.
According to what I've read EPA fuel economy is calculated with non alcohol fuels. Why I don't know...they're not generally available :?
My car runs slightly shy of the EPA rating but not grossly so. I'm inclined to believe the 3% figures.
Out of curiosity I checked with local sea water marina-they too sell gasoline with the ethanol.