My truck is a 2000, auto, 4x4, ext cab, short bed, 4" lift, 315 bfg at's, and stock gears, aem intake and 5" exhaust. And figuring in for the mileage error for the larger tires I've gotten at best 20 mpg. I also have a diablo power programer that on the economy setting only drops my mileage by 2 mpg! So I never ever use it, unless i'm towing I set it on the tow mode and it seems to help mpg's.
But overall, driving at 55-65 mph on a 2 lane highway 100 mile round trip with only slowing down to 45 through a couple towns I can consistantly get 17 mpg during above freezing weather with fuel addative and ULSD. I drive 100 mile round trip to school twice a week. When its below freezing i'm lucky to 12mpg...