I have a 2015 Dually 4wd with Aisin and 4:10's. to answer your MPG question, it all depends on your RPM and load and city/highway.
running 70mph on PA turnpike with 990 loaded and towing a 7x14 v nose trailer I see around 10mpg but this low number is because of the RPM I feel.
Running Rt 13 down through the Delmarva Penninselia towing a 4000 lb jeep liberty 55/65, this is with a red light every couple miles, I can see 12-13 mph if I am not in a hurry.
Now that truck with the Aisin gets you the Highest powered Cummins they produce for that year. With my 990 loaded and towing the jeep, when the light turns green I can accelerate with traffic with no problem. (in fact it will take off faster them most people at a light with no problems).
And the tow/haul switch works very well. Rolling along 55 and the light ahead of you turns yellow, just let off the throttle and truck will slow down enough it almost needs brake lights to come on. The exhaust break will hold down to around 20mph.
But yes the 4:10's might be more than enough. I ordered my new truck with 3:73's to lower the RPM some on my frequent 200 mile trip on the PA turnpike.
