[KamperBob]

It really depends on the vehicle. If you start with a flow mess underneath causing drag you can either reduce it or clean it up. Hucho and Hoerner cover ground effect. On a truck 2WD versus 4WD can make a huge difference on the relationship between ground clearance and fuel economy. It may be more prudent, for example, to lower the former or divert ground air with spoilers, whereas the latter might gain more with a belly pan. I would not lower a truck via suspension mods regardless. Between the cost to benefit ratio and potential negative impacts on vehicle utility, ride quality and resale value seem like good reasons to pursue a different approach.