Jonathan, I absobloominlutely do believe that removing or lowering the tailgate changes the aerodynamics and mileage of pickups. It increases the drag coeficient which makes for poorer mileage.

The aerodynamics of a pickup are such that it essentially carries a "bubble" of air with it in the bed which is almost the same as covering the bed with a tonneau cover. This is design, not an accident. Lowering or removing the tailgate or replacing it with a cargo net, disturbs/perturbs the aerodynamic design and increases drag.

Sorry if it seems counter-intuitive. I used to feel unsettled regarding the little vertical airfoils on the top of airliner wings near the wing roots. How could these things increase performance by exhibiting drag? I finally "got it." Maybe you will too.

Pat