Since you have no way of knowing how much ethanol is added to your fuel, since some states allow UP TO 10%, your fuel mileage will vary according to the "purity" of your gasoline. In those places where you can get 100% gasoline, you will find that your mileage will increase a few MPG.
The facts are that the amount of ethanol in the fuel that you buy varies each time you purchase fuel. Worrying about something that you cannot control is a waste of time. Drive intelligently and you will get the best mileage possible with each tank of fuel, whether it has 5%, 10%, or zero ethanol.