Are you sure you aren't driving differently? I've done several tests in various cars and they all had the same result. For every mph over 60 the mileage drops about 1mpg. I just did a trip from NYC to Boston in my wife's Mini and averaged 47mpg by simply going the speed limit (it was 49mpg for the first half when the speed limit was 55. The second half's 65mph SL brought it down). My wife typically averages about 72mph and gets 37-38mpg.No driving habit changes and the temps have stayed between 30 and 40 for the most part. I'm seeing about 2mpg better from both of our cars.