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Lowering the vehicle will reduce the frontal area somewhat, thereby reducing drag somewhat. OTOH, it may also simultaneously increase the interference drag between underbody and ground, meaning the air has more difficulty and drag escaping from under the car, thereby increasing total drag.
Hoerner's Fluid Dynamic Drag has a section on external stores (bombs, fuel tanks, etc.) on aircraft, which have both streamlined stores and wings or fuselages. Despite this, the flow around the otherwise streamlined bomb gets tangled up with the flow around the otherwise streamlined wing, and total drag may thereby increase by as much as ~60% more than the sum of the individual parts. Ungood. Optimum solution is to increase the gap between bomb and wing to a dimension at least ~40% of bomb diameter, and fair the strut. By way of analogy, increasing the gap between car and road may actually be better than lowering the unfaired car, but raises c.g. and increased frontal area. Better to add fairings and belly pan.
For your application, consider cheap fixes first: Coroplast (used politician's signs for free) under body, in good, smooth application as commonly seen here on EM. Cheap. Light weight. Invisible to casual observer. Then, maybe add some wheel fairings as seen on recent Mercedes et al and also shown previously on EM. Somebody talked about making such wheel fairings from used plastic trash cans, cut diagonally. Works. Resists curb rash. Light weight. El Cheapo. Removable when pesky wifey poo and kids do eeew factor.
The most advanced designs from Germany and Japan seem to have smooth belly, air dam with center section somewhat cut out, and wheel fairings. If you are a good scrounger, you could probably get 98% of the benefit of this research and apply it to your car for very little money, and an afternoon's work.
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