[aerohead]

My opinion is, that the Airtab ought to be used as Gary Wheeler originally intended. On a notchback car.
Stuck at the back of a semitrailer, with nothing behind to affect flow reattachment, it violates the logic of the function.
Even if the wake is 'reduced' in perimeter dimension, it says nothing about the pressure at the separation line, which could be lower than originally, leading to lower base pressure, higher pressure drag, higher overall drag.
I talked with Nish, of The iconic Nish Family, 'Royal Purple Streamliner at one of Mike Cook's 'Shootouts on the Salt', at Bonneville. They have Airtabs at the rear of the streamliner. Nish's explanation for their inclusion had to do with drag-chute deployment success / efficiency, not drag. They've shredded parachutes immediately upon deployment, initiating up to a 12-mile coast, to and from 'Floating Mountain.'