[lunarhighway]

I don't think there's a high pressure above the shape as you drew it, there's actually a low pressure, as air needs to speed up to move around the shape and as such pressure drops, this is why wings work and why racecars need little upside down wings to push them to the ground.

I'm no aero expert, and i hope they'll be able to provide some better explanation on the matter, and correct me if i'm wrong, but there's a few things happeing when a car moves trough air. One this is air getting compressed in front of the car (this is why the radiator is there) as it can't move out of the way fast enough, the other is low pressure being created behind the car. Than there's the air moving around the car.

The ideal shape allowes that all air can move smoothly around the shape and not pile up, and than gradually slow the air down again and bring it back to it's original position. If this shape is scaled up to contain a conventionals car interior, it becomes far to long. but chopping of the tail will still provide an efficient design, because the sharp rear cutoff will ensure that as teh care mores a virtual cone of air stays attached to the back and the airflow comming of the car continues to slide over this cone.

This is esentially what real cars with good aero do as well, and why aero eficcient cars almost always have sharp rear cutoffs, even when they visually don't theres often a hidden straight edge in the taillights or the C pillar.

When you get it wrong and the rear shape is rounded (as with the beetle) there's not much surface for this virtual cone to form on, and also the air comming of the roof will trip into this lower pressure area, creating turbulence.

Therefore a car like a prius seems to have a fairly large rear area where the beetle has a rounded trunk, that looks more organic and aero friendly, but really isn't.