There is a couple factors that could be playing in here. One, winter gas depending on where you live, but it will affect gas mileage. Two, if you let your car idle for extended periods. Also, did you reset the average mileage after each fill up, or is this a 3 tank average? Right now, in the middle of winter, I am averaging 19-20 mpg, city, on winter gas and using remote start. During the summer I averaged 22-24 mpg city and 30+ on the highway.