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Lowering a car or other body in motion increases the drag at a certain point, because the airflow between the body and the ground is constricted. This is called interference drag.
NACA and its various equivalent research groups studied this in conjunction with biplane wing arrangement, where both wings are smooth and streamlined. What is the optimum gap between wings? Too far apart and they lose the synergistic flap effect, where they produce more lift than the sum of their respective lifts. Too close and the interference drag increases, since the airflow beneath the upper wing conflicts with the flow over the lower wing.
Lowering a vehicle reduces the effective frontal area and drag of the wheels, as noted previously by others, However, it also leaves less room for the underbody airflow to efficiently move, so means more interference drag, and otherwise creates suspension and ground-clearance issues. At some point, the benefit of reduced wheel drag is offset and then reversed by the increase in interference drag.
As a practical matter, it seems to me that wheel fairings should be a priority, along with a smooth undertray at the chin. Such undertray should extend back as far as the transmission of a front-engine car, and help induce and smooth the cooling air as it leaves the engine compartment are rejoins the airstream.
Simple fairings of Coroplast could also be made for the front and rear suspension arms, and to plug the parachute-like gap between the fuel tank and the rear bumper cover. Beyond that, covering the entire underbody would seem to have much diminished return on effort, plus carry the complications of heat from exhaust pipes and mufflers, etc..
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