With the right kind of aerodynamics you can get a sort of bubble of air of roughly the right shape. Having a hard surface of that shape (wouldn't be exactly sloped as much as a bubble shape without discontinuities on the trailing edge) would be optimal, but you get 'ginormously bad' by having a sharp discontinuity without any turbulence generation.
That can be either the little nubs of high performance cars, or something like Airtabs that are meant for big-rig trucks. And either way you don't get to increase the mileage by a whopping amount as there are still probably shape issues on the leading edge of the vehicle, plus sheer surface area. To do amazing streamlining the whole thing has to be a bubble, including the front edge of the vehicle.
