With my 2018 389, x15 565 hp I get 6 mpg pulling a light load on a rgn. Between 4.5 -5 with a 80,000 pound gross load in the hills out east.
The Eaton auto shift don't have good reputation. The newer Detroit DT-12 are good automatic because the engine and transmission work together as one. The Detroit transmission can turn on engine brakes. So it doesn't miss a gear. It very helpful in first and second gear, it help speed up the shift. It has GPS with big hill locations. This stops the transmission from shifting gears going uphill. The computers will just let the RPMs go higher then normal before shifting. Also when the truck and engine and transmission all can talk to each other Detroit has Hill Assistance. This holds brakes on at stop lights when to take foot off brake pedal. It stops the truck from rolling back. The cool part is, the second the truck starts to pull the teailer the brakes will automatically release. Eaton can't do because this last I heard. Because Eaton can talk or control any other part of the truck or engine. Detroit also has a very strong engine brake with the auto shift because the transmission can drop one or two gear and bring the engine up to 2,200 RPMs going downhill
Until something brushes against them. Was a red 680, but someone else brushed that colored mirror shroud. H looked my trailer over real well, and blamed the guy who'd brought it back the previous day. I'll take a Cascadia, the plainer the better.