how will ethanol affect my fuel mileage ?

Our unleaded here contains up to 10 percent ethanol.
We now have stations providing both pure gas & ethanol blend.
My wife's 300C will get 1.5-2 MPG better with the pure gas.

I also don't like the Biodiesel.
Put some in a truck stop, lost 3MPG & power on my truck towing a 5th wheel trailer.
It was cheaper.
Next stop, put in the reg diesel, my MPG & power came right back.
This was on a 700 mile trip.