With the 2.7 and automatic in the SWB Tracker, I don't think I've ever managed over 21, which was a few hundred miles at 75mph with an intermittant tailwind.  With 40psi, I'm averaging right around 19.5-20mpg combined (some of each ac, windows, hwy, city).

Although they're lighter, the short trucks are less aerodynamic than the longer ones, and that automatic transmission outright kills mileage.  But I sure do love the truck.