thank you for the idea of looking at bullets. obviously there has been a LOT of research on bullets - more then cars by far.
Very-low-drag bullet - Wikipedia, the free encyclopedia
looking at the pic on the right, no rounding. but they cheated the tail diameter down as much as they had room for.
I went out and spent a couple of hours looking at the details of how to mount a tail to my Suburban.
One thing to keep in mind is if at the rear, any air separation takes a LOT of effort to get reattached.
Therefore, the tail needs to be pretty well attached to the body of the car, and I actually am thinking of making the first 3 inches or so of the tail parallel to the cars direction of travel to ENSURE air stays attached at that point.
if you make it smaller then the body of the car, or leave a gap, I doubt it will ever reattach unless you make the tail 10 degrees or something silly.
The math of wind resistance certainly is appealing. Math says if I am getting 17 mpg now, I may get up to 23 mpg with a good sized tail.
that is a significant savings.