According to this study
http://csus-dspace.calstate.edu/bits...pdf?sequence=1 the "traditional canopy" or topper is good for about a 8.6% drag reduction whereas the tonneau is only good for a 1.15% drag reduction. See page 70. The real winner in pickup drag reduction is the 3-D aerocap good for almost 20% reduction in drag. See the appendix on page 96. I would have figured the larger wake of the traditional canopy would have had worse drag.