Seriously man... think about it. How does someone "easily" get 19 city when the vehicle is EPA tested & certified at 15 city / 19 highway?
I get 13-14 city in my XK and can get over 16 on the highway if I keep my speed below 75mph. Does that mean the Hemi "easily" gets better gas mileage than the 4.7L?
I have a friend with a 4 door JK and he gets about 17 MPG on the highway. He drives a lot (~80 miles a day) so he got a used beat up civic that was in an accident and fixed it for driving to work. So per my experience, which happens to comply with the MPG all of these vehicles are rated for getting, gas mileage is definitely not a reason to swap out for a JK.