Assuming that the frontal area of a car is fixed by the elbowroom and headroom requirements of back seat passengers. Which design of the back of carbody gives lowest coefficient of drag?

The simplest to build is probably a vertical wall behind back seat or trunk. But this looks like it had the aerodynamics of a brick.

And what is the use of a three-box/notchback shape?

Allegedly the best shape (at least assuming no restrictions on total length) is supposed to be kammback profile. Narrowing gradually to about half the maximum area, and then suddenly cut off.

What is the best angle to do the narrowing to the back end?

The new Chevy Volt will use it:
http://gm-volt.com/2008/05/29/production-chevy-volt-to-have-a-cam-back-round-fro...


& the Corvette has used it since the 1969 model.

chornedsnorkack wrote on Sep 17^th, 2009 at 4:04pm:



I would imagine that you would always have a compromise of one for the other, unless what you are after is the perfect balance of the two, but then that would depend on a large number of variables based on the rest of the design. 

Those two cars use it to it's maximum extent for fuel economy while still maintaining reasonable head room for a large variation of body types, while cars like the ones I've mentioned, use the design feature for it's maximum aerodynamic speed/down-force abilities, and feature no backseat at all.

So between the two purposes, the optimum number would be a wide margin apart.