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Time for a thought experiment: Say you have a Jetta with 100 cu. ft. (random number selected for easy in-head math) interior volume. Say the size of the duct necessary to have a positive impact on the rear wake takes up 10 cu. ft.
I maintain that, if you have 10 cu ft of interior to spare, you'd get better aero by chopping the top down until there is 90 cu. ft. interior volume. You'd get less frontal area, and even moreso, less oblique frontal area that is presented to x-winds. You'd get a skosh less weight. You'd get a better fineness ratio. These things by themselves would reduce wake. You'd avoid the extra drag, skin and otherwise, that ducts have.
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