Alcohol has less energy than real gasoline. It takes more gasahol than real gasoline to go the same distance.
In careful study in my own cars, the gas mileage drop is between 18% and 23% depending on the vehicle/engine combo.
I only buy real gasoline these days, no matter the cost/gallon.
A gallon of ethanol has 60% of the BTUs of a gallon of gasoline.
A gallon of 10% ethanol blend has 96% of the BTUs of a gallon of gasoline, so you should get about 4% less gas mileage.
Theoretically.