Lowering helps cover up the wheels and undercarriage. This reduction in frontal area will reduce aero drag. I believe trucks might benefit from lowering even below 50 MPH, because drag is not only caused by frontal area but also how many times that air gets hit passing under the truck. Benefits increase down to about 3 or 4 inches off the ground. Below 3 inches there is no benefit because the proximity to the road surface unless you have a well designed splitter. Covering the bottom with a flat skin will help also. Ideally for a truck, it would good to able to raise the ride height for off road or bad weather. The shape of the front and rear is also very important. I haven't seen a truck yet that was designed for optimal mileage. The need is, to ask, what does nature say a truck should look like instead of, what have trucks always looked like. I see no major improvements in the aero drag of light trucks since my 59.