White Lightning- I agree. I don't think running 80 MPH on the highway locked into 5th gear is going to improve your mileage. But, it may make sense at 65-70 MPH. On my stock rig, mine runs at 1850-1900 at 75 MPH with the cruise on. I've tried different gear settings and it makes the mileage worse.

But everyone should try different setting to see what the sweet spot is on their rig.

I came from a 2005 Ford F250 6.0 that got 18-19 MPG best case running empty on the highway (when it wasn't sitting on the side of the road with the flashers on). I'm getting 17-18 MPG consistently from my 6.7 under the same conditions so I'm quite happy with the mileage I'm seeing. I never expected to get 20+ MPG from a 7,000+ lb HD pickup that tows huge loads with no problem.

 

I don't know what to think about the Cummins powerspec site.

I have a 5.9 but it tells me I need to run between 1900 and 2250 rpm for best mileage or best performance or best mileage/performance. The same engine speed for all three?

Where this is an issue is below the specs it lists all these things that impact mileage and says that Cummins has no control over them. If I run 1900 rpm I am running about 65 mph with or without my 5er. My milage without the 5er is quite good but previous trips of 2500 to 3200 miles I would get about 10.3 mpg towing my 5er. The last trip I made I drove at 60 mph at about 1750 rpm and got about 11.5 mpg. I did downshift on hills if I knew they would cause me to drop rpm or speed. I watched my EGTs and even in 5th gear would back out of the throttle to keep them down.

With the gearing of todays trucks at 3.73 or 4.10 and final drives in the tranny at .73 or .69 it is hard to get close to that 1900-2150 rpm without exceeding speed limits in many states. And many states enforce the federally mandated 65 mph for trucks AND RVs. WA state will enforce 60 mph just like CA enforces the 55 mph. Not saying people don't get away with driving faster while towing but they do pull over RVs.