One thing that is sometimes forgotten about modifying racing technology for the street is that the ground clearance can be minimized for racing but on the road, it is usually kept 4" or more.
A couple of personal data points are: I extended the front fairing on my VW so that it was about 2.5" off the ground (It drug the ground everywhere.) and saw no change economy for thousands of miles.
I built a racing trike fairing from zote foam that was just almost touching the ground and had an open bottom.
It worked pretty good allowing me to maintain 24 mph for a 100 miles on an open road race, I was pedaling. It was several mph above the speed that I could power it without the fairing. Of course the side effect was that the lower lip acted as a venturi and picked up buckets of sand off the road and I ended up with a black face.
Air does not like sharp changes in section. Unless the air flow is cut off nearly completely it doesn't seem to make much difference in drag. I think you would be better off with a lower radius extending to a belly pan.
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