I think that lower energy content of winter gas is only one of the reasons for lower mpg in winter. Others probably include greater rolling resistance of tires in colder temps, greater drag in wheel bearings, longer warmup of the ICE, and other things I probably failed to think of. As I get it, BOTH winter and summer gas contain ethanol these days. The difference is in the blend of molecules in the gasoline so that summer gas has lower volatility than winter gas: Winter vs Summer Gasoline… yes there is a difference « It's just science… and other stuff Other wrinkles: - the "10%" label on pumps really says "UP TO 10%"; typical ethanol content these days is around 6% - in some states the law specifically allows gas retailers to omit any mention that their gas contains ethanol (aren't lobbyists wonderful?), so just because a pump is not labeled ethanol does not necessarily mean that it delivers 100% gas.