I bet you will experience no increase in mileage at all for your proposed modification. When you put a barn door in the bed of your truck the most effective things you can do are to slow down and to feather foot it.
Going from a popup to a hard body truck camper changed our mileage not at all nor did decrease the clearance between my cab over and the bed of my truck by removing one of the riser. Now our cabover rides about 1″ above the marker lights on our cab. The
impact of speed is readily apparent in our F350 Super Duty as you might expect given the exponential relationship between speed and wind resistance.
Steve and Andra
2012 F350 6.2 gasser SRW LB
Fab Fours front and rear in case we run into a rhino
2019 Northstar Laredo SC