Please help with transmission/gearing set up

Why are you wanting to lower the RPM? What are you trying to fix, improve?

IMO you are borderline of needing a tandem with bigger power for what you are doing?

Sure is a nice looking truck.

 

Well you have a couple options.

First of all if the truck pulls the load the way that you like and you feel like you have enough power and it's geared right then I wouldn't mess with your rear axle.

You could go to a 10 speed overdrive you could go to a 13 speed Overdrive or you could go to an 8 LL that has an Overdrive.

The 8ll has an underdrive so you would get a deep reduction in low gear and low reverse. So if you're working on sites somewhere and you want to crawl slower or pull out in a lower gear you can do that. If you are only looking to have an extra gear to shift up into if you occasionally Cruise the highway or however often you do that transmission would do it.

The 10-speed Overdrive and a 13 speed overdrive you would have to look at the model and see what the ratios for the low gear and reverse are and compare it to what you have now. 13 speed generally have a pretty low low gear and reverse and there are some 10-speed like mine that have a really low first gear and reverse. You would have to check on the Eaton website.

The 13 speed really would be your choice if you're on the highway a lot. A 10-speed is fine if you just want the extra gear to shift up when you Cruise.

If you have a driver at all I would avoid the 13 speed.

 

2100 RPM isn't high for an ISC.

 

Kinda my thoughts, especially loaded. Anything under 1850 it will fall on its butt.

 

Normally the shorter the stroke the more RPM they can stand. I have an ISB with an auto. Loaded I turn the OD off and run about 2300 at 55 on state roads. Coming up to a hill I can push the go pedal. Run it on up to 60-65 and it seldom ever has to shift down in central Ga or Sc. I prefer that over the transmission shifting gears all the time. On the interstate, which I seldom run, speed gets up high enough to run in OD.
Little Cummins must be OK with it. It is about to hit 250,000 miles. Never had the head or pan off of it.