Yes, I have several users getting 22+ out of their Cummins over at OB. I question whether a kid is able to do it but adult driven and those who are using them for hotshot service (where added mileage is added $$ at the end of the month) are getting better mileage than the 6.0's and wayyyyyyyy better than the 6.4. I also have one or two who are claiming 30 but I just discount that to a hard knock on the head at a young age.
Conversely, my father drives a 99 5.9L, chipped, exhaust, intake, injectors, tranny mods and he cant get his pig to get over 13 unloaded and its 9-10 loaded (5ver with a pup in tow).
So..........either my dad is the only honest Cummins driver I have ever met or there is something wrong with his rig. He doesnt care, it starts, runs and pulls like a freight train so he just fuels it and drives it. Its going to be on the chopping block this fall after harvest though. Hes having repeated problems with his tranny and he is now looking into other MFG's as he comes closer to replacement time.