3.42 gears for TC

BurbMan wrote:
I wouldn't sweat it, my truck also has 3.42 rear with 6 speed transmission and handled the Lance fine, also pulls the new 13k 5er fine too. Rear axle is more critical with gas engines and older 4 speed transmissions that had wider gear spacing.

Mostly about the transmission (I pulled some info on Ford transmissions. Different models may vary a bit but the general pattern will hold).

In the OP's truck dropping down 1 gear from 10 to 9 will bump up RPM by about 9%.
6 speed transmission would bump RPM by about 20%.
4 speed transmission would bump RPM by about 30%.
3 speed transmission would bump RPM by about 32%.

Additionally, when pushing really hard, 1st gear in the newer transmissions has been able to go deeper the more gears the transmission has. So when pulling a really heavy load up a steep hill, you will be more likely get to the top:
10speed: 4.70-1
6 speed: 4.17-1
4 speed: 2.84-1
3 speed: 2.46-1

With the 3 or 4 speed transmissions, once the truck can't hold top gear, the engine has to run at drastically higher RPM to maintain the same speed.
- If you did significant and heavy towing, a higher (numerical) rear end to hold top gear more of the time and would get you up steeper hills...but running empty, you would be running higher RPM with a loss of efficiency.
- If you rarely do any heavy towing, the lower rear end would give you better efficiency running empty but expect it to rarely hold top gear when towing heavy and on the steepest hills, it might not get to the top.

With the 10speed, it can dial in very close to the ideal RPM to maximize efficiency and power to match load/road conditions regardless of the rear end. Only for the very heaviest loads on the steepest hills might it run out of enough oomph to make it...but that's not even close to happening with a truck camper on a diesel drivetrain.