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One thing I notice about the cab extension in the tuft test video is that there are clearly some vertical axis vortecies produced immediately behind the structure. This can be seen by watching the one tuft on the tonneau spinning. Essentially this strikes me as working to replance one axis of turbulence with another, where the vortex would normally be on axis across the back.
The two things I think need to be changed in this design are:
#1 The open nature of the extension should be closed off with plexi or some other material. This would reduce the amount of area such vorticies can setup in, resulting in smaller vortecies.
#2 Reduce the angle of the sides to nearly on plane with the cab.
The current effect of setting up vertical axis vorticies is at odds with the natural horizontal axis vortex over the broader area behind the cab. The two work against each other. And since it's pretty clear such a design is not going to extinguish such vortecies entirely, it makes more sense to reduce the number of them and clean up what's left as much as possible.
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