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I can't get my head around how the TVargo Cap would be a dramatically better Cd. I still think a more squarish cap would have more usable space and be a bit simpler to build with common materials.
Something that doesn't get talked about as much as it should is "grand vortex formation" when ever you have a low pressure area next to a high pressure, you get the air wanting to move from high to low, and since you're hauling down the road, this is going to create a spinning motion of the air.
We must remember, the air is just sitting there (Ideally on a calm day) and we are ramming through it with our vehicles. The energy required to move the air out of the way, and then the energy we put into it to make it swirl about, is all felt as drag. The more the air is set in motion behind the vehicle, the more energy it takes to do that. So if you have a constant fairly high velocity swirl going on behind you as you drive, this requires energy to sustain. If the air behind you settles out quickly, and returns to its calm state in short order, than you have not put much energy into it and therefore you will feel less drag.
So it is these pressure differences in close proximity that we are looking to avoid in order minimize stirring up the calm air, and thus reduce drag.
Swirled ice cream cones GOOD, Swirled air BAD. My basic rule of thumb.
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