IIRC Popular Mechanics magazine published an owners survey of Durango owners back in the late 1990s, and the 5.2 got 0.75 MPG better than the 5.9 V8. I believe both were in the 13's.

I also seem to remember that the 5.2 V8 version of the Jeep Grand Cherokee in the PM survey was reported by owners as getting about 2 MPG better than the 5.2 Durango.

I wondered why?
Was it weight, aero, 4wd desgn, JTEC software, etc