The more aerodynamic things at the front, the more aerodynamic resistance (drag) you will have at the rear. Airtabs at the trailing edges of openings, like at the rear of the cab/sleeper/trailer, rooftop air conditioners, toolboxes, the cargo box, etc., will minimize or eliminate the drag. Airtabs won't make a leading edge of anything more aerodynamic, though. Just the opposite actually, as its physical presence increases wind resistance.
The sun visor acts as an airfoil, or a scoop, and traps air between the visor and the windshield. Putting Airtabs on the leading edge of the visor won't prevent it from trapping air underneath the visor.
Imagine the following tilted down in the other direction, as a visor on the front of the truck:
The "Relative Wind" gets trapped under the visor by the windshield, as there is no "trailing edge".
If it was a wing, as above and below, the air would flow past the airfoil and the Downwash passing by it would create high pressure underneath and low pressure above, creating lift (which is what makes airplane go, and what causes the drag behind the truck, since the truck itself works in the same way).
But because that sunvisor only allows air to freely pass overhead, there is no downwash to create low pressure above, so the pressure above the cab stays the same, and the pressure on the windshield, especially in between the windshield and the visor, rises due to increased aerodynamic wind resistance. Placing Airtabs on the leading edge of the visor will have no effect on what happens underneath the visor. Same with the leading edge of the roof, since there is no drag there and there is already positive air pressure flowing over it, anyway. The trailing edge of the roof, if there is a space between the roof and the box, that's where to place them.
The visor itself makes the front of the vehicle less aerodynamic, and short of filling in the space between the visor and the windshield, there's really nothing you can do to increase the aerodynamics of the visor. Well, you can remove it, of course, but then there's the tradeoff between comfort and safety, and aerodynamics.
I put a bug shield and side-window rain guards on my Sprinter, which resulted in an immediate .5 to .8 MPG hit on fuel. Adding Airtabs gave me that right back, and then some. But the rain guards are worth every penny, and the bug shield, while not even close to being 100% effective, is far more effective at keeping bugs off the windshield than if it wasn't there (sometimes driving through Texas or Arkansas, I wonder if it's working at all, tho
)
If my bug shield were a few inches taller, Airtabs would help, since it would eliminate the drag immediately behind the shield at the hood, but the Airtabs are too big to fit. On a big truck's bug shield, they may very well fit, and would do the job. But not with a visor that traps air between it and the windshield.
You might want to try placing the Airtabs underneath the front of the visor, though, if there's room. Not all the way across like normal, but maybe one in the center and then one or two on either side. That may disrupt the airflow enough to force the air out to the sides and around the truck quicker, as the Airtabs themselves will create higher pressure directly behind them, and there will be lower pressure to either side of them, forcing the air to the side and around the windshield. If you place them all across the visor like normal, then the air pressure would be the same all across, and you'd be right back to where you started.
In other words, you want to prevent the air from being trapped under the foil in the first place, or provide a more aerodynamically efficient means if getting the air out of there.
