Aerodynamics & Diverting Wind Resistance ?

Just so you know from my experience. I had a 89 D200 with a 360. Money got tight for me driving 500 miles a week with $4.00 a gallon gas. I made a perimeter frame out of 2x4’s flush inside the box and covered it with 1/4” plywood. I went from 16 mpg to 17mpg. I rattle canned it flat black and even made it pivot at the back of the cab to lift up for easy access. I wonder if I could have picked up a little more with a smooth paint finish like a coat of ploy urethane.