Sorry. But I'm not buying this. Are your averages hand calculated or are you reading the lie o meter? My 03 dodge 3500 with the 5.9 averages 19 on the highway and 15-16 city.
While I agree that the cummins platform will last longer than the average driver will drive it( with proper maintenance). Your Mileage seems a little high to me.
i dont find shortround to be the kind of guy that doesnt hand calculate. i have also seen mpg go up after break in and in higher mileage. so i dont tend to doubt him. but if he said something like 28mpg id be thinking twice.