[jhillyer]

What we want at the rear is as smooth a return flow, after the craft has displaced the air from the roadway. Rearward turbulence sucks the car rearward as drag.

I read another's example here about car height of 8" and 4". Getting lower to the roadway helps peel air over and to the sides of the craft, reducing the high pressure stackup under the car, which we know among more subtle losses can dangerously lift the nose at speed, and a little lift can pitch the nose enough to gulp a catastrophic plunge of air pressure and flip the vehicle nose over tail.

We want the air to flow to the sides of the craft, and we'll avoid lift forces over the top and rear of the craft. What we allow to flow over the craft can be trapped near the rear trailing area, costing turbulence, but creating another stack of high pressure for localized downforce. But once at high speed with great ground effects and little flow over the craft, we don't need the high wing or air dam at the trailing edge. Winglets at the leading edge of the craft help trap flow toward the sides, further helping vacuum the craft without later vacuum at the upper trailing edge where we can further lose downforce. -- just babble.

-edit-
Rear diffusers are generally decorative, beyond helping shape the rear flow to reduce turbulence. However, it would be possible to increase vaccum under the craft if ducting were facing aft and high enough or sideward enough into a slipstream, thereby drafting air from under the vehicle, but if done high, again, costing more turbulence. This should work.