[skyking]

that 12~14" inches above the cab is a cab mounted rounded fairing idea, to smooth the transition from the sloping windshield to the horizontal section. Nothing will go in there and it is fixed to the truck itself.
sorry if you guys misunderstood.
It was an attempt to slow down that air a bit. As Phil has mentioned from time to time, the surface velocity above a blocky cab is 90 MPH at 60 MPH forward speed. The whole purpose is to get that speed back down to 60 as it leaves the rig.
If I can mitigate that acceleration just a bit it will help.
As far as the sail area, the crosswind pulls you around, not pushes, and it pulls the hardest on a square edge.
That bump fairing will not add to frontal area of the combination, but it will have really gradual radiuses on the front and sides.