Last winter my gas mileage was around 6 mpg less than it was through the summer. Not only are the batteries and the oil cold, but they oxygenate our gas for cleaner air in the winter. I know that drops mileage in some cars (my 96 chevy s-10 dropped a couple mpg).

I've also been seeing my mileage falling lately with the temperature. My last tank was my second best at 44.6 mpg (46.3 to the cars computer) with mild temperatures, lows in the 40's and 50's, highs in the 70's. During the last week our high temperatures have dropped into the 50's and it's rained a couple times. Now the car's computer says my average is 42.8 on this tank. I've probably done a couple more short drives on this tank, but some is definitely the cold.