[Piwoslaw]

The general shape of today's cars looks like a reversed teardrop, but there are small details that would kill aero when going backwards. One of them is the small notch at the rear of the roof. Its purpose is to separate airflow so doesn't stick to the rear glass, increasing lift. In reverse this notch would cause turbulance all the way down the roof.
Another detail is the subtle Kamming of the rear half. Most cars designed in the last two decades have a roof which slopes slightly down, with max roof camber above the driver's head, not in the rear. Exeption: PT Cruiser.