We installed Air Tabs several years ago and they did as advertised. The rig is more stable in gusting side winds and almost no buffeting from trucks.
I did notice a reduction in the cloud of spray generated in wet conditions and a cleaner toad as a result. It is apparent that there was a reduction in the turbulent air behind the rig. There was a down side for us, the rear view camera is now in a smother air flow area so the camera lens now quickly becomes wet during the rain.
Quantifying the mpg gains is difficult, however there does seem to be an improvement of 1/2 mpg or less.
We installed the Air Tabs for improved stability, so any fuel improvement issues are appreciated but secondary.