The 3.43 Rear end

Wiley75 wrote:

Peak torque of 850 Cummins = 1700 rpm
RPM at 70mph with 3.73 = 1750 rpm
RPM at 70mph with 3.42 = 1590 rpm

There's more to this story. You don't always need peak torque. I'm willing to guess the torque that's available at 1590 RPMs is enough to handle most situations. But when it isn't, you'll have to downshift, and you need to consider where your RPMs will be when you do that. If downshifting one or even two gears pushes the engine out of it powerband, you won't be able to harness that torque multiplication when you need it. I would choose the rear-end that provides useful torque in the greatest number of gears at the speeds you most often drive. Then, on the road, it becomes a matter of selecting the appropriate transmission gear to match the situation.

-- Rob