I think the mileage also varies by road condition(i.e. rain, snow, elevation changes) and temperature. In the last two months, mine has dropped from 24.5mpg avg to 20.5 avg. I feel this is probably normal due to the roads now being covered in snow and the temp going from the 60's to the -20's. My '09 gets 33 or so on a long highway run over reasonably flat terrain at 65 mph in warm weather.