Do you mean higher octane when you said higher grade? Higher grade gas might give you slightly better mileage, higher octane will not. Poorer mileage in winter is due mostly to the winter blend (more ethanol) of gas. In very slippery conditions, tire rotation with lack of traction can have a mior affect also. As for the colder therfore denser air, I believe the computer adjusts the injectors for that. Cold also affects tire pressure. It'll be less in cold weather and less pressure in tires also has a very minor affect on mileage. Of all the above, the winter blend gas is the biggest culprit. Driving habits affect mileage more than all the others listed above.

technically no, but it steals power. so if you want to go the same performance, you will give it more gas, and therefore get worse mileage (i think)

but i have gone 160 miles and im at half tank.
that'* with 20-30 minutes of idling every day

its been proven to.(i like to watch mythbusters). 2 SUVs were tested one with the Air Conditioner on the other with it off and the windows down and the truck without the A/C on went about 10-20 more mi. then the 1 with it on

I don't right away put the heat on full blast when starting the car, after the car is warm I'll put it on Medium you can say..

But look at what speed they were going. You start pushing 65, 75, or beyond mpg, which has more drag? A/C or open window?

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Just remembered, running the defroster kicks the compressor on usually too.

yea mine dropped down from about 3 and some change to maybe 230? maybe? i might be rounding up

and the speed of both were about 45mph. but i thought that drag holds the car back. and with the windows open air is rushing inside messing with the aerodynamics and in doing this the car would have to work harder to maintain the speed thus using more gas. but in the study the A/C running car lost the challenge. so that why i said what i did about the A/C

hmmmm, I don't know how much city driving you guys are doing, but I have a VERY heavy foot and still get almost 300 out of a full tank, and my O2 sensor is pretty much dead :?

Ony my car if you keep the heat low (read: no flow through the heater core), YES, the gauge does move up quicker off the cold mark.

HOWEVER, once it starts moving up and I crank the heat up, the cold stuff in the heater core circulates into the system and my gauge goes down a bit... might be a bit of a moot point. Nowadays I just keep the system on full heat full time.