IIRC the new car window stickers for the v6s said 16-19 mpg for the manual and 17-20 mpg for the auto. YMMV
The best I have ever seen was 20.1 mpg on a trip from Flagstaff, AZ to Gallup NM; empty trip. At the time I had 285/75/16 with 3" fagtech lift. Since then, I have added campershell with roof rack, lowered the truck to a 2" lift (OME), and down sized to 265/75/16 (E-RATED ). The best I have seen with this setup is 19.3mpg hwy, though I had 300lbs of hunting gear in truck. In town I average 17-18 empty truck.