I recnetly travelled through Iowa and stopped to get gas. After I filled up I realized it was the 15% Ethanol stuff. It was cheaper that the 10% stuff. My milage dropped 2 MPGs!
A gallon of ethanol has 60% of the BTUs of a gallon of gasoline, meaning you will get 40% less mileage from a gallon of ethanol compared to a gallon of gasoline.
That means you will get 4% less mileage from a 10% blend, and 6% less mileage from a 15% blend.
Theoretically.