Whats your engine now? A 4 liter will get better mileage than it.
A lot of people say that the ranger will get about 20 mpg, im not sure highway or city. But then, thats without too much skinny pedal. A lot of it is in your driving style. So yes, if you swap a good well running explorer engine in then you should have gains over the 3.0/4.0(kinda depending on the 3.0 though, since some of them get great gas mileage((for a truck))).