[ever_green]

I get best results (both power and mileage) with more alcohol in the tank up to 15%, but any more E and mileage starts going downhill for me. With winter E15 I can really crank up ignition timing and actually lean out mixture and make more torque, specially at low RPMs where my engine knocks with regular straight summer gas. As a general rule and from dyno observations BSFC improves dramatically with higher ignition timing at 14-14.7 AFR and i have my engine tuned to this fuel mixture ratio below 3000 RPM with over 7* timing advance compared to stock. I try to use E10-15 gas rated at 89 octanes. I understand E10-15 can actually harm your mileage if your car is not tuned for it though.

Also part of the reason why your E10 mileage is worst than your E0 is because you are comparing winter time temperatures with summer time. Intake temperatures have a lot more effect on your mileage than having or not having 10% of ethanol. Another reason could be high mixtures of cold start aids in winter gas that evaporate in your tank such as butane.