Funny how even TrailerLife.com even states that around 1,800 is the best rpm for fuel economy. I guess they are wrong too along with the Cummins engineers.
Diesel Tech Q&A: Axle-Ratio Selection"When it comes to fuel economy, running empty around town or when driving where speeds are less than 55 mph, you’d see very little difference in fuel mileage between the two axle ratios because the transmission will never get the engine rpm high enough to be a factor. When towing, whatever gear keeps engine rpm between 1,800 and 2,000 rpm will give the best fuel mileage and power because that is the rpm window where the Cummins 6.7-liter makes peak torque. With a little time, you’ll find the engine’s “sweet spot” with the trailer you are towing.
The difference in towing mpg between the 3.42 and 3.73 gears at highway cruise speed, under the same conditions, should be less than 1 mpg because of the slightly higher rpm running 3.73s. Vehicle speed actually plays a bigger role in fuel economy than engine rpm. That’s because towing or unladen, mpg drops exponentially at speeds above 60 mph, which is when aerodynamic drag (Cd) really begins rearing its ugly head related to pickups."
