These are the first diesel engines that I have owned so I really don't know much about them but I would think the Cummins RPM would be about the same as the 6.2 though.I was just thinking what is the max governed RPM of the Cummins? The 6.2 was around 3000 I believe, if the axle ratio is to low the truck may not have much speed to run the hi way.
My uncle made two vacation trips out to California towing a fifthwheel camper and he said he had no problems running 75 on the expressways.
I don't know what rear gears the Dodge had but my truck has 373 gears so I don't think I'll have any problem with highway speeds.