Sorry it's been so long since I could post up some data. Here's a years worth of MPG data:
http://spreadsheets.google.com/pub?k...H8RY-8lAK1BtZQ
Preliminary data view looks like there is a slight increase in warmer weather, but nothing dramatic. Perhaps 5 miles at the most. Given that, this is through all types of driving, and is more indicative of whether I was doing city driving or freeway driving. During the summer months I was on the highway more than not, and during the winter months around town more. That alone would account for the decrease in MPG.