I understand that Ford has two different rear ends in the 450. Mine didn't have the cover plate on the back and the shop I was talking to could not do the conversion.

I'm getting an avarage of 8.5 pulling 15,000 lbs and 12 to 13 empty with my 4.88. I try to travel between 60 to 65 mph and agree that 65mph is 2500 rpm. My truck happens to be a 4x2 which might make a difference. I can tell you that the 4.88 in my 450 makes a lot more noise than my 350 with 3.73 did because of the difference in the rpm's, both trucks are 08's