Both FFV vehicles, by energy content E10 to E85 is about 25-27% less, actual mpg drop about 15-18%, if E85 was roughly 20% cheaper I used it. If you check out EPA website they basically rate FFV cars at 1/3 less with E85 than E0.
I wish 5-10% drop, a North or South dakota University study with non & FFV showed some getting equal to better mpg with between 20-40%E. It's
http://www.ethanol.org/pdf/contentmg...our_gallon.pdf
Highlights of the study here, search a little more to find the whole study. Basically comes down to trying different blends with every car to figure out if one works better. Blender pumps would make it way easier.