Moses, thank you for the detailed answer.
I used the RPM calculator in the link you provided. I first did the baseline calculation using stock (~33" tires) with 3.42 gears, and the Aisin's final drive of .63. The resulting RPM for 65 mph (probably the ideal cruising speed for reasons you already noted) was 1426 RPM, which seems quite low for the 6.7l Cummins. For curiosity's sake, I also tried 75 mph and got a RPM of 1645, which seems closer to the engine's peak torque sweet spot (1700 RPM). I'm guessing the FCA engineers set up the gearing to allow for 75 mph at low RPM's? Even if so, I do agree that with a big truck, aerodynamics will certainly degrade fuel efficiency at that speed, even if the engine RPM is at an efficient value.
Using that as the baseline, I did two more calculations:
35" tires with 3.73 gears. At 65 mph, the RPM was 1466; again low, but it seems close to OEM specs. At 75 mph, the RPM was 1692, which again puts it very close to OEM specs and the peak torque RPM.
35" tires with 4.10 gears. At 65 mph, the RPM was 1612 RPM. At 75 mph, the RPM was 1860; this seems a bit higher than what I would want for long highway cruises, no?
The stock gearing RPM calculations kind of surprised me. With stock tires and gearing, it almost seems as if you'd be lugging the engine at 65 mph. But if my only concern is to put the engine back into OEM RPM ranges with 35" tires, the 3.73 gearing does seem to do that.
I will also note that on this hypothetical truck we're discussing, there is a strong possibility of putting on a lot of aftermarket gear (bumpers, winches, auxiliary fuel, ect.) to the tune of 1k lbs. If that was the case, I'd imagine 4.10's would make more sense for moving all that extra weight, in addition to any extra payload and trailers, correct? If kept at a mostly stock base weight, would the 3.73's be a better choice?