As proven in the Alpina C1 the front of your car is just as important as the back of your car. Taking the e30 body and doing nothing to the back, only the duckbill on the front, and getting a remarkable 105.72 mpg Imperial average. This is remarkable because the e30 is not aerodynamic at all. It is very boxy and rigid, not a single smooth edge on that car.
The amazing mpg was achieved in 1981 by the Aplina team in a 318I and a stock engine. There is not a huge amount of info about this car but here is a short Jalopnik write up about it.
This Little Alpina BMW Makes Hybrids Look Stupid
Duck bills on insights anybody?