the X has a higher numerical diff ratio to help make up for the extra weight I believe. I also know that if you leave the shifter in MM, the convertor does not lock on my 05 X. If you leave the cruise on a certain speed, say 70 mph on the highway, then shift from drive to manual mode, and shift back from 4th to 5th since it drops to 4th automatically when you go to MM - it will register about 200-250 more RPM's in 5th gear when you are in MM vs. Auto.