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There is a thread here but i can't link it due to only having 3 posts yet. I will quote it instead - it's in my bookmarks.
To quote....
1. Narrowing the body at the front and rear,
2. Side window angle (22° and 30°),
3. Length of front bumper,
4. Angle of grille (0°, 12.5°, 25°),
5. Angle of hood/bonnet (5°, 7°, 9°),
6. Angle of windshield (25°, 30°, 35°),
7. Angle of rear window (30°, 35°, 40°, 45°, 50°),
8. Inward angling of rear pillar,
9. Angle of trunk/boot (4°, 8°, 12°),
10. Geometry of rear spoiler.
The effect of each of these elements on the drag coefficient was tested, then the propotional change in Cx (Cd) was calculated. Here is an ordering of those effects:
1. 46.5% - Angle of hood/bonnet (element #5)
2. 17.0% - Angle of grille (element #4)
3. 11.7% - Narrowing the body at the front and rear (element #1)
4. 11.5% - Angle of rear window (element #7)
5. 3.9% - Inward angling of rear pillar (element #8)
6. 1.9% - Relationship between rear window angle and spoiler
7. 1.7% - Relationship between windshield and rear window angles
As the above list shows, the hood/bonnet angle has the largest effect on the drag coefficient Cx of a sedan shaped vehicle. Increasing it from 5° to 9° reduced Cd from 0.52 to 0.47.
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