[gumby79]

Originally Posted by Piwoslaw

Originally Posted by guyed

if you look to the left of the cab, you can see the effectivly sheet plastic 1/4 circle vertical deflector.

??
The only semicircle thing I'm making out to the left of the cab is the spot / convex Mirror On the passenger side. Or are you referring to what the arrows are pointing to on the front beside the grill above the head lights?
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Does this help by deflecting towards the rear air that would normally be pushed to the side, thereby gaining forward momentum (sort of like a sail), or is this outweighed by the penalty of extra drag?

Primary purpose in my eye , is the your first part of your question. To divert air to the rear. That would otherwisewould go into making a larger dynamic frontal aera.

That visor has a much smaller outlet than inlet. It takes energy to compress air.

+1
Agreed compressing AKA working cost energy. But with the right geometry the parasitic loss can be reduced for a net gain as dimminstrated
HEAR.
+Cd*-A can be LESS FUEL
Dose the extra work (+Cd) offset the reduced dynamic A(-A)
For a totle reduction of CdA? That depends on how you design it. I'm willing to put money these pictured on this truck reduced the total.