Ram towing mileage

I bought my 2019 Ram 2500 Diesel almost 2 years ago. 17-19 mpg around town and 21-22 on the highway empty. Towing the 12k lb 5th wheel, I was at 10-11 mpg rolling down the highway.

I'm not talking about the Cummins mileage compared to the Hemi here.... But rather an observation of these 2 quotes from above in this thread as they relate to the transmission gearing......

After reading this thread last week, and seeing this one, I limited my trans to no higher than 5th gear (I'm liking that little button switch on the steering wheel).
We went 4 hours away into the mountains of Northern Pennsylvania and picked up 1.5mpg. Towing there I got 12.5 and coming home, I got 13.
And yes, it kept my RPMs up a couple of hundred and I noticed much better throttle response when going into an incline.


Might get better mileage towing in 5th gear instead of 6th.

Could be bogging the engine in overdrive. Need to let the engine come up some more in rpm where it will make more efficient power.

So that makes me question this:

My 2019 Ram limited, 8 spd, 5.7, the best I've ever had mostly freeway driving is 17 mpg, around town it's about 14 mpg, towing my 4000lb RV on the freeway it drops to 10 mpg, with it in "tow haul " as recommended by the owners manual so that's O/D off.

Unless the 6 speed behind the Cummins acts different than the 8 speed behind the Hemi, I'm not so sure the tow/haul turns off the OD..... Previous to this last trip, I simply put it in tow/haul mode and went down the road. It still goes into high gear. So it does not turn the OD off, it just holds the gears longer. At least in my truck, anyway. The only way to keep mine out of high gear (OD) is to limit it to 5 gears with that button switch.

So engine choice aside, gearing matters.