Gas mileage varied more this year than last.The best we did was between September 5 and September 22, when we averaged 35.6 miles per gallon. Most of that was highway driving -- to and from the airport for travel to the 2010 NSGIC conference.
Two time periods were tied for worst gas mileage, at 29.2 mpg. One was between January 28 and February 3, when it was fairly cold (with a little snow). The other low-point came immediately following the high-point. Between September 22 and October 5 we also averaged 29.2 mpg, but I have no idea why.
The metrics above are for the period between December 31, 2009 and December 31, 2010. They are derived from my gas mileage log, which is maintained as a Google spreadsheet. I also record mileage using Matt Haughey's fuelly.com, which tells me that my long-term average mileage -- since the summer of 2008 -- has been 32.1 miles per gallon.