Around 60mph seems to work best for me. My 5th wheel weighs (according to registration) 10,400lbs. I can get about 11 or 12 with this load. But when trying to keep up with traffic on I40 (75mph), I drop into the single digits.

 

I agree with about 1800 running at 68 with 3.73's and get right at or a little more than 13 MPG pulling 5th wheel and Boat. Total gross at 21000.
The wind kills that though

 

63 mph or so. As long as rpms remain at or under 1900. Different for hilly country (dropping a gear to Direct; zooming down hills to make the next one and trying to keep constant, overall).

 

2000 rpm for me, I belive there is even a service letter from cummins about this for 24v

 

Grade and wind are the main determiner. flat and still= 67mph@ about 1850

Grade and/or wind= out of overdrive and 60 @ about 2100rpms. This is what I usuallly due to in new mexico and colorado. Its easier on tranny, lower on egts and a good displacement of stress.

I think 1900-2100 rpms underload is a good area overall.

 

Don't quite know the sweet spot for mine yet. Pulling about 14.8K fifth wheel. Only have 3000 miles on truck so far. Everything says the mileage will increase. Getting about 11 MPG now at about 60-62 MPH.

 

Don't know sweet spot yet. My new truck only has 3000 miles. Everything says the miles per gallon will increase as approaching 10K miles. Getting 11 now at 60-65 MPH.