chip makes a big difference with the fuel trims, its like my tbird, even with the 3.73s it can still see 26-28mpg cruising and 19-24 around town, but without the chip it was down 2-3mpg then what im geting now
it could also be related to the 20mph difference in speed. aerodynamic resitance can increase exponentially depending on the shape of the truck, so maybe try unloaded cruising around 50, see if it goes up, but then again maybe the addative helped
your guys miledge seems really low though, my 6.9 without o/d could get a best of around 16 or 17 cruising, and it was going 60-75mph usually, all it had was an exhaust and a pump running more fuel, so the newer di motors getting worse then that is wierd