i have mentioned in other threads that i cut the lower half of my bumper with a circular saw in order to reduce the 'parachute' effect from my rear bumper. another alternative is the oem diffuser from honda, which is really just a peice of plastic blocking the 'parachute'.
cutting the bumper is better because it eliminates some of the wake behind the car as well. ultimately the best setup would involve cutting the rear bumper and adding some panel to smooth out the air flow.
anyway some pics:
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