When I had my 00 Dodge with the NV5600, towing the same trailer, it would always do better in 5th at a slower speed especially with the 3.55 gears. 55mph would show 13.5 on the instant fuel mileage thing in the overhead. That only told me the engine was not loaded as much with 235hp and 460 ftlbs of torque, anything helped. In Sixth at 63-65 the truck consistanly got 9.8 to 10 mpg. Needed some more rear to run in OD. Did not have an EGT reader in the truck then. I really think that 5.9 with a set of 4.10's would have done good with my trailer.

Dave

 

At highway speeds (they can vary) and depending on the load (uphill, wind, drag, weight; they can vary) between 1700 and 2400 rpms is where I keep it. If they vary from that I feel I'm doing something wrong.

Every scenario is different, different trailers, tires, wind, road, etc.

I don't go below 1700 rpms because a cummins mechanic told me not to , it's too close for comfort in the bog zone if I'm not paying attention.



I'm not passing 2400rpms unless it's a large grade and large load.... and I should probably slow down .

 

Loading the engine was my main concern, because it seemed the coolant ran a little hotter when the engine was turning higher RPM's in the lower gear.

 

You can see this from time to time because the head flow gets significantly more restrictive at higher RPM. Sure, you're making boost pressure up in that range, but the FLOW isn't any higher. If you're loaded so heavily that you're nearly choked flow, I can see how higher RPM might increase EGT, even in 5th.

The elevated coolant temps are partly due to proportional increase in restriction at higher RPM.

JH