Took a 3000 mile round trip in my buddy's 97 ram a few years ago. His was a 1500, 4x4, extended cab, 5.2/auto. We were picking up something for me so I was footing the fuel bill. 75mph got us 13mpg. 55mph brought us up to 18mpg. That's about a 40% improvement. The rest of the trip the speedo wasn't allowed over 55. This testing was done when we were driving through the flat lands with the cruise set.

I would imagine the 5.9 to be a little worse, especially if yours has had the death flash that knocks a bunch of timing out of the calibration.