[aerohead]

The above paper was published by a team at Texas Tech University who had done model and full scale wind tunnel testing on a Ford F-150 full-size bed pickup.
In lieu of a full-on aero-shell they found that a cab-wing extension combined with GM's patented 1/2-tonneau cover produced drag reduction within 2% of the aero-shell.
The truck still had basically full utility of the bed,although,of course,one wouldn't be able to secure anything nor protect it from the weather.
My friend John Gilkison did this to his Toyota pickup and it lived up to expectations.
Also,a blister,atop a 1/2-tonneau,which mimics only the back half of an aero-shell is good for an 8% mpg by itself,same as the wing/1/2-tonneau combo.
Only down side is rear visibility.