Join Date: 3rd May 2014
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From a science point of view, I believe it does the following:
The air flowing over the front of the car is relatively laminar, which is efficient for drag. However, when the body narrows, the air flow tries to pull away from the car, starting boundary layer separation. At this point, it's actually more drag efficient to "trip up" the air and send it into a turbulent regime (counter intuitively).
It's the same effect as the holes on a golf ball.
From a non-scientific point of view, they look crap and give you at best 1/2 to 1 mpg improvement at high speeds (IMO)