The optimum subsonic shape is a hemisperhical nose with a long tail tapering to a point ( like an ice-cream cornette on its side, with the dome of ice-cream heading into the wind ).

The K-tail is an abbreviation of that impractical-for-cars long tail which gives most of the effect for much less effort.

The Tatra T77 was an attempt to implement the optimum form though even then the tail was somewhat abbreviated:

http://tatra77.com/