Well mine is very strange also. I've had my RTL-E for a little over a week and honestly haven't had anywhere really to go--barely have even 150 miles on it yet. But every time I take a little trip to the store or the like, the mpg keeps increasing. Today I drove less than ten miles and the mpg crept up from 15.3 to 15.6. It does this every time I drive it. It doesn't go down, just slowly up. Of course I'll be tracking it as I go along and right now it doesn't make a big difference to me, so tracking it by fill ups should be a little more accurate!