Wires on old airplanes (cylinders) were huge drag creaters. As was said they have drag many times of an airfoil. On top of this a lot of simple fairings on aircraft have been known to give racing aircraft 50+mph increase in speed.
But what is speed with out control. You build an airfoil like they did with the AMX-3 and guess what; the wheels come airborne around 100mph.
If the reduction of drag is the only thought, then you want to taper the rear like an airfoil as long as possible and delay the separation to avoid turbulence.
I have given this a lot of thought to aerodynamics in my car and is something I want to play and test when I get it together. I have been watching guys put tassels on cars to test aerodynamics which frankly were too long. They get out of the boundary layer and are influenced by other air which makes the test less then useful.
The approach to my car was not as much about a reduction of drag as it was keeping it planted on the ground. I laid the radiator down, closed off all air going under the car. I ducted the air out the hood.
I also built the belly pan to help smooth the air under the car. A diffuser would be the next step but I doubt I will get that far.
What I would like to do is take what I have and test the effects. To do so my plan (to start) is taking information from 2 things. One is simply putting a variable pot on the suspension with a display to look at (and other info) real time and a computer to log the info when I feel like playing with it. The info would be the suspension height compared to speed. It would be one way to measure the effect of the wind.
Second I want to datalog pressure from specific locations at speed.
The scoops on the side will not pull air in on their own but fans to oil coolers and such will pull the air in. I am curious how the fans pulling the air in will delay the separation. I also have an idea for VG's which will keep the air against the car possibly making the vents work without fans.
The golf ball dimple concept is a neat idea which has been played with from time to time. I think car builders have a hard time getting past a car which looks like a golf ball.
On a truck they tested they also found a truck with a netting for a tail gate or a tail gate up got better gas mileage then with the tailgate down. That went against everything we can envision but it shows the complexities.
It's all fun. If you didn't have anything to do when you got up in the morning what would you do?