Friction is ultimate energy waster and contributor of finite range.
I originally wanted to build a super fancy, aerodynamic, teardrop shaped truck bed cover (see below), but that would not be very practical.
I want one that won't get in the way of me swinging by Home Depot on a whim for a couple sheets of OSB. I compromised for a quick and dirty flat cover. Using the last scraps of plastic, (from the sheet used for the air dam), I hastily built a cover that could withstand an 80 mph head wind (freeway driving conditions).
As luck would have it, I drove to work the next day during some of the worst winds that Davis County has seen in over 20 years. As I drove southbound through Centerville, through the high wind corridor, the 90-103 mph east wind gusts made quick work of my truck bed cover, tearing half of it off. Watching electrical transformers light up the early morning sky while trying to avoid flying debris and rubberneckers looking at semi trucks that had flipped over on I-15, I somehow made it in to work.
Here is my sort-of-repaired bed cover. Wind is not partial to ugliness, only form.
While the bed cover is good at protecting the contents of the bed, the jury is still out weather it actually improves the aerodynamics or not.
The MythBusters TV show actually did some testing and found that bed covers don't help fuel efficiency at all. They also did some other tests with the tail-gate up, down, off or a meshed tailgate.
While their tests were extensive, they only performed them at 25 mph and 55 mph and only on one style of pickup truck. I still think there is some merit to a bed cover.
The ideal bed cover would be an aerodynamic shaped one like this.
Isn't that a beauty? I don't know who made this but he should go into business.