I know these are old threads regarding gas mileage. I've been searching these forums for a thread on whether gas mileage goes up or down with a seasonal change. I live in Maryland, a rather moderate climate, and have noticed that my mileage has dropped from around 19.6 in mixed use during winter to 17.0 mixed use in summer. I have a 3.0i. I suspect that the gasoline is highly oxygenated in winter as a smog reduction effort and not as highly ox'd in summer. It's a theory but may explain the seasonal decrease. Anybody have any thoughts?