[whitevette]

Rather than get tangled in the many parameters which affect fuel consumption (previous posts go into everything from tire pressure to engine management "tricks"), let me throw one more over-looked idea into the brew.
Everyone is aware of lower air temperature causing gas mileage to sag.
One just doesn't get the same gas mileage in the winter as in the summer months; it is noticeably less. Blame has been placed on everything imaginable.
Cold air is more dense than warm air. But why must this density increase (or temperature decrease) create such a drastic change? I'll propose a simple argument: It's impacting more molecules (of air) as the vehicle is driven through the air. This higher viscosity air must be "forced apart" by the front of the vehicle with an increase in throttle ... thereby requiring a bit more throttle.
I can hear the cries now: "Absurd!" "Crazy"! ...let me offer the following reason.
Any pilot will tell you airplanes develop less lift when the surrounding air is hot. Summer weather calls for a longer take-off roll (or landing length of runway) than cooler weather ... or, higher altitude airfields (where the air is thinner) need longer runways than lower elevation airfields. These are very drastic differences. Doesn't the same air affect cars & trucks?
Remember the K.I.S.S. principle....