I'm in a 2008 B4000 automatic 4x4 with 4.10 limited slip. I get about 19-20 on the highway and get about 15-16 in the city. I improve my mileage by regularly maintaining my vehicle and by not driving like a jackass everywhere I go. I have a rigid fiberglass tonneau cover on the bed that I'm going to take off (it came with the truck from the dealer) and try that and see if it affects my mileage. Mythbusters says they don't do a thing for mileage.
It's a pickup truck...not my old 240SX. I drive nice, smooth, and consistent...no flooring it from light to light slamming on my brakes, etc. Just put on some nice music and take it easy for the ride. When you see traffic slowing ahead, let off the gas early and coast down before hitting the brakes...the traffic could start moving before you get there and save you from having to stop and accelerate back up to speed. I rarely exceed the speed limit. My optimum speed is about 60-65 on the highway and I don't ever go above that unless there's some big reason. There's a huge drop off in mileage between 65 and 70 in my truck. It just gets worse above 70. At about 75 cruising, I think I get about 14 or 15.
Oh, truck is brand new and getting that mileage with stock 16" tires and wheels, no mods