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I directed most of the air over my car because of its limited ground clearance. I didn't want to redirect so much air under the car that it was compressed and restricted by the limited space under there and develop a high pressure zone under the car. In outline, my car is a catamaran with an air splitting "pod" running along each side enclosing the wheels and a smooth underbelly promoting laminar flow under the car and expanding (diffusing) at the rear as the underside of the boattail rises upward.
The ideal shape for an ultralow Cd vehicle would provide a high ground clearance with a smooth underside with enough space under the vehicle to avoid interference with the air flowing under the vehicle. In essence, a low flying aircraft fuselage, which is the approach they take with many ultra-streamlined solar powered racing vehicles.
With an SUV, you have more ground clearance than my car has, so you could direct more of the air under the vehicle than I can by using a higher stagnation point, but you would still need to direct much of the air over and around the sides of the vehicle. The thing you want to avoid is directing so much air under the vehicle that it piles up, loses its laminar flow, and develops upforce under the vehicle. And needless to say, you would need to have a smooth transition from the front splitter to the totally smooth underside to keep the air flowing smoothly and laminarly under the vehicle.
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