[aerohead]

Have you watched the 'THINK FLIGHT' YouTube of the aero-modded Subaru Impreza ?
He tested at 65-mph, with no 'events.'
I highly recommend some sub-structure, firmly attached to the car.
On the CRX, the cardboard mockup was fiberglassed to become the final boat tail.
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On the car or truck, the ride becomes as with a limousine. Rock solid, and unflappable in crosswind, gusts, or bow-shocks off 18-wheelers.