[brucepick]

Ok I see your point there.
It's a good explanation of why the Elio's back has the shape it does.

I think the design at this point has a pretty good Cd overall. I had thought you were finding some degree of fault with the shape but I think that was a misunderstanding on my part.

I want to point out here, I'm pretty sure the recommended rates of taper are maximums and not prescribed rates. A passenger jet fuselage has a much gentler taper rate than nearly any road vehicle, and of course has a very low Cd. So a designer isn't forced to taper faster than they want to. At least not by the laws of aerodynamics.