Some time ago I covered some elementary tips for truck aerodynamics. I began to wonder about making a homemade tonneau cover. After discovering a patent for GM's half-bed tonneau, I built such a tonneau out of weak particle board and screws.
Lately I've been chronicling the fuel savings I've experienced with the tonneau. My first run didn't get great mileage but it seemed like a slight improvement.
From now on, I'll be keeping track of the fuel economy using GreenHybrid. See the gas mileage my Ford F150 XL 4.6L gets. Before the half-bed tonneau--which is really a flimsy prototype at this point that is sagging due to rains--I got slightly lower economy (4-7%).
For those needing a baseline, the trucks are rated at 15/19. Most people experience worse economy than what the EPA publishes. Mine personally got around 16.5/18.9. I'm hoping to achieve 17.5/20.5 with the tonneau.
Even with all this, though, I pale in comparison to the Accord, which clocks in at around 27/35.