Something has boggled my mind from the beginning of this thread. You claim that your girlfriend (with a heavy foot) gets a consistent 25mpg out of her Sport Trac. I find that hard to believe, although not necessarily impossible.
A Ranger is lighter and has a smaller profile and can be found with several smaller powerplants than the Sport Trac. As long as you have properly sized tires and proper gearing, and you don't drive it like a race car, you should see decent fuel mileage - at least as good as a Sport Trac.
If economy is a priority, you have to be willing to resign yourself to the fact that a 4cyl won't have V8 power. And you also have to keep in mind that trucks in general aren't designed for economy, they are designed for being trucks.