Thoughts on Proper gear ratio

Thing is, gear ratio won't affect the speed you pull the hill, that's where the engine comes into play. Lower gears are going to give you more grunt to get 140k lbs moving when you need to lift off on an 8%. 2.XX rear you're going to be putting a ton of stress on the powertrain. Lower gears theoretically are stronger too (more teeth in mesh at a time).

 

For super B Detroit actually recommends 1600 rpm at cruising speed for the DD16 for best performance, but then fuel economy suffers.
It's all a compromise.
The problem with the DT 12 is your next gear down is direct, it would operate in high range a lot like a 15 SPD over if you've ever drove one.
If you're on the Coq a lot the 3.58 may be a better choice, if you're only ever on 2 lane, 3.70 may be a better solution.
Everyone says this motor pulls good down to 1000 rpm.

 

If you put the ''tire'' size sure, eg
a 235 80 22.5 is only 37.3 in high

 

I agree with you here.

You have to go down hills too and the 4.10 over the 3.90 rear gear will have an effect on the ''holding'' power . THE jake [or not] will work better in other words saving brakes.

The torque curve of the engine figures into the equation also as has been mentioned.
With the 3.58's I would spec disc brakes IMO

 

Yea a lot don't realize it has nothing to do at highway speeds just getting you moving

 

I agree to some extent that low RPM equals efficiency but cruising at 1400 with your foot into it vs cruising at 1650 with light throttle and low boost probably won't result in better economy.

 

Now that's one experiment I've done lol. Almost 3/10s better mpg running at 1600 at 65 vs 1400

 

round your way people haul 140k on 235/22.5’s?

 

Sorry I don't agree.
It's torque that pulls the load and the lower rear gear transfers more torque to the wheels so pulls the load easier in any trans gear in theory.