My truck is a 2013 Eco w/3.55 rear end.
Tires are now 33" e-rated Michelins vs 32" stock which gives me about the same as a 3.44 rear with stock tires,
Fully loaded with my travel trailer and junk I'm just over 8k lbs.
I can pull the grade from Heber to Strawberry with that load, at the posted limit, at 2700 RPM except for the very last bit at the north side of the pass where it drops a gear and bumps up to 3300 RPM.
If the trailer didn't have the wind profile of a haystack I'm sure it would not downshift at all.
It's a joy to drive in the mountains unloaded. With cruise on it won't break 1700 RPM crossing Soldier Summit and the 36 gal tank gives me about a 600 mile range when a cruising around the west desert.
As a comparison to a big cube gasser, I drove my setup to Vernal along with my brother in law in his '95 chevy 2500 w/454 CID. His trailer and setup was 2k pounds lighter and I'd always pull away from him on the hills. We both averaged about 9 mpg for the trip, but I seriously doubt he'd get anywhere near unloaded mileage, city or highway.
and another thing.... If you are the kind that has to have the big V8 rumble, this is not your engine. It is blissfully quiet. Pay close attention if you are test driving one since many people correlate power and acceleration with sound.