This should have it's own thread IMHO as weight is THE big fuel user in ~98% of driving (Urban) with aero a distant second UNLESS it's very light or doing double duty as part of the load bearing structure.
All this 'Look my Boat Tail' etc overlooks this fact of physics an equal %age of the time.
Building an Ultra Light-Weight Car, Part 1
Building an Ultra Light-Weight Car, Part 2