This report confirms what I suggested in another thread, that there is a specific zone behind the cab after which it makes sense to attempt to establish more laminar flow off the rear of pickups. A longer bed, with far less impact from cab length, will benefit more greatly from aerodynamic aids for this reason. I would argue that if one had a bed length nearly equal to the height of the cab to the bed, it would not make any significant difference, tailgate, mesh, tonneau, or whatever because there is virtually no opportunity to re-establish a laminar flow that close to the cab at anything but the slowest of speeds where the impact is nearly nil.
In short, I think it could be argued that longer beds net better aerodynamic possibilities and shorter beds provide less opportunity for improvement. The length of the cab is relatively insignificant.
I just watched part of the re-run of mythbusters on the pickup tailgates. you may loose .02 cd but if the 80+ lbs tailgate is removed wouldn't the mpg go up some if 100lbs = 1mpg. they didn't factor the weight reduction just focused on air disruption.
It looks like Ford's lightest F150 is 4670 lbs. So knocking 80 lbs off that nets 4590, or a 1.7% weight reduction. One would be lucky if that translates to even half that much in fuel economy improvement considering all of the time most vehicles spend either cruising or stopped idling.
now i know what I'm about to say has no testing they behind it or any 100% accurate Truth, but i know with s-10's that if you have a toneau cover that you will see a 1-2mpg increase even tho the thing weighs 80lbs(fiberglass with some metal subframe) several people that ive talked to have said they picked up a 1-2 mpg increase with it, not to mention the security and ability to leave somewhat valuable stuff fin the bed. Ive been thinking about making mine out of a steel or aluminum frame with sheet metal as a main covering. i calculated it out and it would be around 60-70 lbs minus the hinges and gas struts to keep it up. cost about half the price of a store bought one.
and yea a truck is meant for hauling stuff. throw oh lets say a 50 lb bag of sand in a car and your gonna notice it, throw it in a truck and it will be like a soda can...