Why not have a nontapered air channel that has the same dimensions at front and back? That way, you are not forcing the air to be compressed as you move forward. I think having a smooth channel down the center underbody of the car would provide stability at high speed, but if you arch the top of the channel upward toward the front you are creating a lift in the front that could cause an accident at high speed and/or powerful headwind. If the channel is wider at the back and front intake is underneath the engine, you create a partial vacuum which can help hold the road at high speed. If the air channel has powerful suction using something like a 200 mph leafblower powered thru the car electrical system, you will be sucking the air out of the bow wave area which I think would provide some efficiency boost. I think the passive method may be best though. (Leafblowers will violate all sorts of noise ordinances, unless properly muffled using a vortex straight-thru muffler.)
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