MetroMPG - Unless their engineers are anything like their management staff...
Person w/ the Toy PreRunner - If you have stake pockets in your rig:
- Put tight fitting boards in them, at least as tall as the top of your cab.
- Get a chalk line, and learn to calculate the angle of a scalene triangle based on it's right angle size. (Or get a protractor)
- Tap a nail into the board at the cab-line on the front,
- Tap a nail into the rear board at a level that creates a 13* angle from the front to the rear
- Snap your chalk line between those two points, and cut them off on that angle
- Same on other side
- Frame off the 4 or 6 boards in your stake pockets so they're all attached to each other, and won't move around
- Get a canvas tarp, or something waterproof that doesn't look too "redneck" (if you care about looks)
- Attach it to the frame, starting at the top and cleanly folding it into itself to create the taper down to the tailgate, stapling/brad nailing it in place as you go
- Test
- Record results
You should see some kind of drag improvement by doing this, as it effectively creates a boat-tail of your bed, one of the easiest things to do to a truck.
You will, however, notice less winter (snow and ice) traction at speed, due to the lack of downforce caused by the (add term here) effect, which creates an oblong air-swirl in the bed of the truck that helps to keep the rear planted at speed (see: Mythbusters episode on "tailgate down mileage savings")
I'm not sure if that's a misconception though (the traction thing). Although at first glance, it seems plausible, since it's a high pressure area.
Anyway, give that a shot, you might like what you see. If it works out for you, it's easily removable and replaceable, should you need your entire truck bed for something, and you could easily make an aluminum button frame and cut/sew a canvas or other material for it.