Ethanol Blended Fuel and MPG: What E10 and E85 Do to Fuel Economy

Ethanol Energy Content

Ethanol (ethyl alcohol) contains approximately 76,330 BTU per gallon of lower heating value, compared to 114,132 BTU/gallon for gasoline — about 67% of gasoline's energy density. When you blend ethanol with gasoline, the mixture has lower energy per gallon, requiring more volume to travel the same distance.

E10 (10% Ethanol — Standard US Gasoline)

Standard US gasoline contains up to 10% ethanol (E10). The theoretical MPG penalty is approximately 3.3% vs pure gasoline. In practice, the difference is 2–4% depending on engine calibration. Most modern ECUs are calibrated for E10, so the penalty is already built into EPA ratings.

E15 and E20

E15 (15% ethanol) is approved for 2001+ vehicles and is appearing at more stations. Expected MPG penalty vs E10: approximately 1.5%. Some automakers have cleared E20 use, anticipating higher ethanol mandates. MPG penalty relative to E10: approximately 3%.

E85 (85% Ethanol — Flex Fuel)

Flex-fuel vehicles (FFVs) can run on any ethanol blend up to E85. The MPG penalty of E85 vs E10 is approximately 25–30%. However, E85 often costs less per gallon than E10, partially offsetting the fuel economy difference. Compare fuel cost-per-mile, not price-per-gallon.

HHO and Ethanol Fuels

HHO systems work with ethanol-blended fuels. Ethanol already contains some oxygen, slightly reducing the combustion improvement from HHO's oxygen component. Net HHO improvement on E85 is typically lower than on pure gasoline.