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Coupes can have a problem where they form too efficient an airfoil shape with a positive angle of attack. This makes them create lift and the lift vector is tilted back a little from the vertical resulting in lift induced drag...
This is why coupes and kammbacks should not curve smoothly off to as near the floor as possible and should try to have clean edge seperation at about the half height mark, with a reasonable "trunk" type shape to make sure the air is turned to the horizontal before separating... New Beetles for example have horrible drag... until fitted with a spoiler...
So, pick your coupe carefully and be prepared to correct it with an appropriate "whale tail" or extra lip on the trunkette if it has less than about a couple of inches of surface to turn the airstream.
Supercars and exotics have a lot of upkick from their rear diffusers to try to help this problem and to try to alter the effective angle of attack of the body shape. Some saturn coupes have body lines that indicate the designer was thinking right, aiming to create more upwash at the back, I'd think those ones do better than the others.
That pretty pic above, the before and after pic, you'll notice that it changes the effective angle of rear airflow by shortening the effective airfoil shape by a couple of feet. This kinda tips the lift vector forward, stopping it making drag.
Modern vehicles seem to be bearing this principle in mind more. The mods I described for Marvin may have detrimental effects if he's not kicking up enough air from underneath. Lower down the sides the aero will be designed to move air upwards....
Now there is also an effect that creates downwash that will mess up the best intentions here. This is similar to tip vortices on an aircraft wing and because the "span" of a car is very narrow it can have significant effects. What happens is that the air will try to roll round the sides off the top of the vehicle. This is why lipped side skirts can be effective, they stop that air getting back under the car. Other measures might be effective also. For example roof fences are seen as "lift killers" for safety on race cars that go sideways, but I think they would also have an effect on reducing the "tip vortex" type down wash from the roof, improving the rear airflow by keeping it from being sucked down too much behind the vehicle, increasing the effective angle of attack and making induced drag. I keep wondering actually if there's actually something to be said for 1957s fins in that regard. Not how they were implemented then, but how something looking somewhat similar but actually functional might be used these days, it would look I think a lot like "winglets" on a learjet. Come to think of it, those Superbird fins probably had as much effect on downwash prevention as they did in "keeping the spoiler in clean air"
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I remember The RoadWarrior..To understand who he was, you have to go back to another time..the world was powered by the black fuel & the desert sprouted great cities..Gone now, swept away..two mighty warrior tribes went to war & touched off a blaze which engulfed them all. Without fuel, they were nothing..thundering machines sputtered & stopped..Only those mobile enough to scavenge, brutal enough to pillage would survive. The gangs took over the highways, ready to wage war for a tank of juice
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