[freebeard]

ecomodder.com/wiki/Pickup_truck_aeroshell

hecomodder.com/forum/fuel-economy-mpg-modifications.php#c2

Pickup truck aeroshell/fastback bed cover - permalink
Pickup trucks have high coefficients of drag. Fortunately, they're also ideal platforms for adding this highly effective aerodynamic modification.

A number of EcoModders have undertaken this task, recording highway fuel economy improvements of between 13-20% compared to an uncovered bed.

They can be built in such a way as to retain the usability of the bed as well.

Examples / Info / Threads:
Commercially produced aerodynamic pickup bed cap
Phil Knox fleet - 34 years of aeromodding
Nissan OEM pickup aeroshell / bed cap