We put AirTabs on the truck after our first trip out. (4 weeks out). We really didn't notice that big of a difference with the mileage, (didn't really keep track, but we get about 10.5 now), but really noticed a big difference with stability. We no longer get blown about when getting passed by a semi, and the truck seems more stable in crosswinds.
We applied then on both sides and across the top of the box.
2007 KW T600 with a Cat C-11 and 10 speed Ultrashift. 10.5mpg on flat ground and no wind. We usually drive between 62-65. Truck is governed at 65. Mostly driven east of the Mississippi.