The epa ratings for E85 and Gas on flex-fuel vehicles very closely match the theoretical loss due to the lower energy content of ethanol. Following that same math for E10, you get a 3% reduction.
Example:
2009 Ford Crown Vic
Gas: 16 city / 24 highway
E85: 12 city / 17 highway
GASOLINE lower heating value: 116,090 BTU/gal
ETHANOL lower heating value: 76,330 BTU/gal
E85 = 82,294 BTU/gal
82,294 / 116,090 = 70.9%
16 mpg * 70.9% = 11.34 mpg
24 mpg * 70.9% = 17.0 mpg