[basjoos]

The Cd would go up because you'd have quadrupled the size of the recirculation eddy trailing the car.

When I first built my prototype boattail, it was cut off at the point where the back edge of my hatch glazing is now, then later I added a tail cone to complete the boattail (see pics in my Photobucket site). I got a noticeable increase in coasting performance when I went from the partial to the full boattail. There's a reason you don't see subsonic aircraft flying around with kammbacked boattails, its called increased drag.