East coast of BC;) Cranbrook. Near the Alberta border.

I'm surprised at how far the factory numbers are off. My cummins will definitely be grunting but the 4.10 will help

I've got the 3.73. It's a compromise between fuel mileage and take off power. I'm not sure I wouldn't go 4.10 on the next truck. Maybe even the 4.30. I'm not sure the gas mileage would be all that bad. I get 21-22 on the open hwy and 16-17 in town. Now...When towing? 9-11. Since we won;t be driving with the 5th wheel ALL of the time, it might not be all that bad. If anyone has 4.10 (or the new 4.30) on a Ram, I'd like to hear about your mileages...

Our new 2018 RAM 3500 DRW we ordered is a 2x4 with the 4:10 Axle Ratio, Asian Transmission with the Cummings HO Engine.
The Cummings engine turns right at 2000 RPM at 70 Mph on the RAM.
Our previous 2012 GMC 3500 DRW 4x4 had a 3:73 Axle Ratio in it and turned about 1800 RPM at 70 Mph.
When starting off on a hill, the GMC worked a little to get the weight moving and always had a little driveline shutter when starting off on a hill.
The new RAM with the 4:10 will start off on a hill with ease and has no driveline shutter. Of course the 2018 RAMs Cummings engine has a higher torque rating (930 Ft Lbs.) than the 2012 Duramax (765 Ft Lbs.)
Fuel mileage towing with the RAM is about the same as the GMC at 10 to 12 depending on hills and speed.
Running Solo with the RAM my fuel mileage has been around 18 mpg depending on hills and speed (of course the RAM only has a few thousand miles on it so it may get a little better with more miles) so there is some fuel mileage penalty compared to what I would have gotten with the RAMs standard 3:42 Axle ratio over going with the 4:10 Axle Ratio, but I sure love towing with the 4:10. The RAM handles our Redwood with ease, starting and stopping is no issue at all, plenty of power when needed and the ride of our RAM with the factory optional "Air Assist" rear suspension is awesome. (it is not just air bags stuck on a standard axle spring assembly, the "Air Assist" option changes the rear Leaf Spring Pack down to only two leaf springs so the unloaded and loaded ride is great and each side has individual air bag automatic level / pressure control and an "Alternate Ride Height" button on the dash for level trailer towing)
Our 2012 GMC Duramax 4x4 with the 3:73 Axle Ratio did not get any better running solo as around 18 mpg was about the best I ever got with it.