The 1991 Ford Ranger XLT 4.0L (automatic) I had wasn't too bad. It got 15 mpg city and 20 mpg highway (18 combined mpg). The 3.0L V6 doesn't get any better gas mileage than the 4.0L V6. Most of the ones for sale cheap have 140K+ miles on them (kind of like Miatas!).
I had issues with the 4.0L engine overheating in the summer even after replacing the radiator, fan clutch, thermostat, hoses and water pump. The 4.0 is a lot of engine to put in such a small truck. I was thinking about putting in electric fans when the Cash for Clunkers deal came up.