get yourself a nice confortable shoe with a thin sole, and a large fresh egg from a local grocery store. Use a quality rubber band from a local BMW dealer and put the gg under your right shoe with a rubber band. When you push the accelerator make sure the egg stays in one piece. Guaranteed 6-8mpg improvement with all the performance chip. Stock chip, even better. Make sure you know how to brake with your left foot, otherwise you'll have to buy a whole rack of eggs.

seriusly the way you drive can do a lot to the performance. I had 11mpg and 21mpg in the same car on the same route, but the time and style was lot different each time...