Just to add some real-world data, I put the shell on my pickup in winter (mainly so I can leave skis &c in it). Here's the tank mpg as I've been tracking it. Don't keep track of dates, but the first tank is probably in November.
# m g m/g
1 16.243 404.8 24.92
2 15.168 383.7 25.30
3 15.440 402.8 26.09 <- Cold & lots of snow starts
4 14.197 351.7 24.77 <- Shell on about 2/3 of tank
5 15.350 405.5 26.42
We finally got good snow towards the end of tank 3 (and also very cold for here temps) so mpg goes down on tank 4 'cause I was driving up the mountain in snow/4WD almost every day. On #5, the weather has warmed up and I'm back to not skiing every day, so tanks #2, #3 and #5 would be much the same driving pattern.
So from this it looks like a cab-high shell might be marginally better than an open bed, at least on my '88 Toyota.