Hatches are notoriusly bad aerodynamically compared to sedans, all else being equal of course...Originally Posted by pizza_boy
Due to the abruptly truncated rear body, the laminar flow seperates in a big way as soon as the airflow reaches the end of the car, and creates a lot of turbulence behind the car. Sedans allow the flow to pass more gently downward over the rear window onto the boot lid, then the flow seperates at the boot lip. The perfect aerodynamic shape for reduced drag is a 'teardrop' shape (blunt end forward), and sedan bodies more closely resemble this than do hatch bodies.
With good airflow over the car body, the area of drag inducing turbulence behind it tends to be about as large as the area at the extreme rear end of the car, which is much smaller with a sedan than a hatch. Of course there are exceptions, like those hideous notch back Volvos, which have loss of laminar flow at the roof lip and at the boot lip.
My understanding is that rear lips don't do much if anything to reduce drag, but if properly designed do slightly reduce rear lift, but most are just bling...
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