Ran across a patent site that reminded me of some guys who ran 200+ at B'ville with an early S10. Seems that wind tunnel testing showed that a hard bed cover over the rearmost 50-60 percent of the bed showed less drag than a full bed cover. It looked odd, but it worked. Might be easy to test.
Ya, I read something somewhere, can't remember where, but, They tested trucks with the last 18 inches or so of the bed covered. and they had a lower CD than tail gate down, tailgate removed, full bed cover, and shell.
It seemed kinda weird, but yes that would be an easy one to test.
correct me if i'm wrong but could it be like this:
since because of the sharp cutoff of the cab, there will always be a wake behind it and the bed cover will only provide a surface for the air to reattach to near the end. the first part will be covered by a pocket of turbulent air
the bed contains a volume of air that's at ambient pressure and stable, while the wake behind the cabin is of lower pressure than ambient and turbulent, if they're fully separated by a cover things will stay that way, but if these two bodies of air are connected that might result in the pressure of the wake to become closer to ambient and/or to be less turbulent.resulting in a shorter wake = less drag. the rear bed cover still needs to be there to provide a surface for the air to reattach to and to keep the air in the bed apart. sort of like a rear defuser on a sports car but with a different air source.
that's my theory... don't know if there's any truth in it...
Here's my guess on what is going on which builds upon what is already suggested: With the bed fully covered, the turbulent air immediately behind the cab effectively rolls opposite the direction of travel. The lower part of that vortex tends to be slowed by contact with the bed cover, creating successive smaller vortecies the further back you go until the airflow reattaches near the rear. With the front part of the cover removed, the vortex created immediately behind the cab is much larger, but because of the cutoff at the leading edge of the bed cover, it does not propagate past the cutoff - effectively providing a single large vortex that requires less energy to sustain. (More like a big ball that just rolls in place behind the cab.) Without seeing test results, I would bet that the ideal length of open bed closely matches the total depth from the top of the cab to the bed floor.
In essence, this rolling ball of air provides a Bernouli effect to more smoothly route the outer air back down to the bed cover.
Snax, I took a few minutes to measure my Colorado crew cab with short bed. Height from bed floor to top of roof is ~39". The bed length is ~65". If the open area of the bed is 39"--the resultant rear cover is only ~26" long and ~60" wide. I don't know if that's enough length to be effective. Of course, I could extend it a few inches past the tailgate.
Wonder what effect low vertical fences on the sides would have. Gonna have to think about this over another cup of coffee. Sounds like an underpowered ProStock truck.
It was Gale Banks who built the S-15 that ran 222mph at B'ville in 1990. You can catch some glimpses of the partial, rear bed cover on the Banks Power web site. I'm going to drop him a letter to see if there's any info available.