[darcane]

This article from Hot Rod magazine is enlightening:
Car Aerodynamics - Hot Rod Magazine

Using wind tunnel testing, they were able to reduce their already modified Camaro from .229 Cd to .201 and a similar, nearly stock Camaro from .497 Cd to .292.

Top 5 Easy Aero Mods That Almost Always Work
We pressed Eaker to give us clues about simple aero tricks for typical musclecars-stuff that will work nearly every time even without tunnel testing. The delta between Cd numbers is often referred to as "counts," or thousandths of a point, so the difference between a Cd of 0.250 and 0.200 is 50 counts. Eaker gave us some rough estimates on how many counts of Cd you might improve when these mods are applied to a typical older musclecar. Try these next time you're at the track.

Lower the ride height: "Dropping the car-front or rear-will always reduce drag." On our Camaro, raising the tail 1.5 inches changed the Cd from 0.201 to 0.227.Improvement: At least 20 counts per inch

Block the grille: Eaker's tip is "Always get air around the car rather than through it." Stopping air from entering the grille has shown dragstrip e.t. reductions of a tenth or two even on 120-mph cars. This will also reduce front-end lift, and when the front end raises due to lift, drag increases even more. (Bonus tip: Stiff front springs can also help prevent front-end lift). Grille blockage is more effective on older cars than on newer ones with sealed radiator areas, and obviously, it can only be used for short-duration events due to reduced engine cooling.
Improvement: 15 to 30 counts of reduction in Cd and 50 to 100 counts in reduction of Cl

Add a front air dam: "You can usually get 90 percent there just by adding a dam straight down from the front bumper, just like the one on your Camaro." Even Bonneville guys ask us why we have that big barn door on the front of the car, but keeping air out from under the car both reduces drag and neutralizes lift for solid aero gains.
Improvement: 20 counts less drag, 50 less lift. The taller the car sits, the more important the air dam is.

Seal the back of the hood to the cowl: This is another area that's less critical on newer cars that are already well sealed. Cowl-induction hoods have a high-pressure area at the base of the hood. Air does not travel out the back of the hood; it is forced down into the engine compartment. That air must escape through the car, causing flow disruptions and drag.
Improvement: 10-20 counts less drag, 50 to 75 in lift

Remove the outside rearview mirrors: Factory mirrors hanging off the doors are almost always obstacles to airflow. The '94-up Chevy Impalas and Caprices are notable exceptions; on those cars, the Cd actually gets worse when you remove the mirrors.
Improvement: 10 to 20 counts of drag



Aero Stuff That Doesn't Really Matter
In addition to our list of five tricks that almost always work, here are some things A2 customers might want to try that are really a waste of time.

Wax: Contrary to what you may read on your favorite message board, well-waxed, smooth paint is no more aerodynamic than the worst spray-can, flat-black primer job you can imagine.

Golf-ball dimples: They do not work on cars, regardless of the scale of the dimples, unless your car is a 1.68-inch-diameter sphere spinning through the air with no ground plane.

Taping seams: Rarely if ever are body-panel seams so large and misaligned that smoothing them with duct tape will make a measurable difference in Cd. We tried it on our Camaro, and it did nothing.

Smoothing rivets and hood pins: The removal of minor surface burbles, such as rivets (remember the Howard Hughes movie?) and hairpin-type hood pins shows no measurable improvement in Cd.

Dropping the tailgate: On a pickup, lowering the tailgate does not usually reduce drag. If you are racing a truck, know that extended cabs and crew cabs are more aero than regular cabs.

The biggie: windshield rake: According to Eaker, "Here's a myth I can bust. Once the windshield is past 45 degrees of rake-and many stock cars average like 60 degrees-you will not see an improvement from laying it down at an even steeper angle." We proved this on the Camaro, building a hugely sloped "windshield" out of foam core. It did nothing.