Gas mileage of ethanol-added gasoline?

[N1LAF]

BUT, and this is a big but, the CO2 from the ethanol part is carbon neutral.

Octane (n-dodecane): C12H26 + 18.5 O2 → 12 CO2 + 13 H2O 18.5 x energy units

Ethanol: 2 C6H12O6 + 12 O2 → 12 CO2 + 12 H2O 12.0 x energy units

Because of the reduced mileage, you have to consume more fuel to go the same distance, which results in more CO2 release.