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Lowering helps to reduce Cd when you have a rough and turbulent underside that you want to divert the air flow away from. Where the added drag of diverting the air the additional distance to the sides of the vehicle is less than the drag that the same air would have produced had it gone under the vehicle and interacted with the rough underside. If you have a totally smooth underside, the ideal would be raise the vehicle high enough to reduce some of the ground effect compression of the air passing under the vehicle, as they do on some of the solar racers and with the early Aptera prototypes (before they lowered them to handle the mechanicals when they switched to FWD, raising the Cd). The solar racers have very skinny wheels to keep the exposed wheel surface area to a minimum.
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