rough calculation based on Diesel efficiency would make it 20 to 30% better mpg, so if you're getting 16mpg with your 4banger then driving the same way would make it 19 to 21 mpg (again, roughly) with the diesel. if you get it geared better tires you might see some more improvement but i doubt that would be too much (i'd say 25mpg as an optimistic estimate).
if your Jeep is stock tires and no lift that would result in increased mpg accordingly (based on where you start).
