If you do a lot of aerodynamic testing like Porsche did in the early 1970's you might be able to come up with a large Gurney Flap at the rear of the car that lowers Cd and raises top speed by also increasing down-force (Ducktail).
My guess is that just guessing at it will have unpredictable results.
However, the general idea I see is that anything you do to fill out the template, the better off you are.
Link to related thread - see numbers on chart I posted.
http://ecomodder.com/forum/showthrea...s-34840-4.html
More images in my rusty old dysfunctional bucket:
http://s184.photobucket.com/user/kac...?sort=3&page=1