The 10% alcohol would have negligible effect on mileage, if they actually blended it with 87 octane regular. Unfortunately they blend it with 85/6 octane and count on the alcohol to bring the octane rating to 87. To some degree a higher octane fuel should theoretically give better mileage, because the knock sensor would allow slightly more advanced ignition timing. I would imagine if the knock sensor wasn't effective, our cars with their relatively high compression ratio would ping like crazy on regular. I have always run 87 in my car and have never heard audible spark knock, even living in a relatively hilly area like I do. I have tried 89 and the improvement in mileage was so slight that it was a wash against the extra price.
04 300M, Lux Group, Rims, & Roof
Give a man a fish, and he will eat for a day. Teach a man to fish, and he will sit in a boat all day drinking beer!