Quote Originally Posted by Thmstec View Post
I thought it was worse in the winter

I thought the "winter" gasoline mix was worse for mileage. And I was under the belief that warm air coming into the intake was better for efficiency than cold air? Although I guess the A/C being off would help...
Optimum air temperature for engines is somewhere in the 50* - 70*F range, according to the former automotive engineers at Car & Driver. So you should get your best mileage then. Temps above or below that range should decrease mileage.

I am personally convinced that mileage goes down when it's really cold (like below freezing) - maybe because the cold air is slower to combust? I don't know... pure speculation on my part.

If you live in a state that has a different grade of fuel in the winter, that may be another cause of lower seasonal mileage, but I thought that the low-vapor-pressure fuels used in the summer in some areas decreased economy, and that the "normal" fuel used in the winter was better for fuel economy.