[lunarhighway]

for good aero you basically want a teardrop shape with a long tail. less ideal but still good you could have a shorter shape with sharp rear edges that allow for a clean sepparation of the airflow.

the beetle has none of this, the air separate very early and will become turbulent,
also the wheel wells will cause the air to sepparate and not reattach to the body.

i'd assume the air starts to sepparate much sooner, i think at the top of the rear window, and also the wheel fairings will greate much more turbulence than the image shows