Re-gearing for the Cummins

[martinique78]

So i have been driving my Fummins Excursion for a few weeks now with no real issues.  However on the freeway i'm turning some pretty high RPM's at 70-80.  Like around 2700 rpm.  At first I thought I wasn't getting lockup but I installed the lockout switch and an LED to show when the PCM was commanding lockup.  all was working properly even without using the switch.  So my next though was to check the door tag for the axel ratio...4.30!!!!!!  holy crap that explains it.  Now I know why from a dead stop that thing feels like it could rip the rear end right out from under it. 

 

What would be a good ratio to switch to?  I have the 2001 24V and the 47RE trans. I'm currently running 315/75/16 tires but I plan to lift the truck and run 36's on it in the future.  Is it even worth changing the gears if i'm gonna be turning larger tires soon or is that ratio still too big for even that setup?

Edited June 3, 2017 by Mopar1973Man
Edited the title

[Me78569]

Here ya go

 

 

 

[JAG1]

You could just change over to the 36's before any regear an see how it does.

 

 Like to see that rig. Did you put the engine in yourself?

[martinique78]

1 minute ago, JAG1 said:

You could just change over to the 36's before any regear an see how it does.

 

 Like to see that rig. Did you put the engine in yourself?

That's what I was thinking too, it doesn't really bother me that the RPM's are that high.  I just know I could get better fuel mileage if I brought the gearing down.

 

Yeah I did it all myself.  Destroked mounts, switched the input shaft on my Ford transfer case to the dodge input shaft and mated up perfectly to the 47RE.  No change in drive shaft length. Used the V10 radiator and the intercooler from a 7.3. Once I sold off the rest of the parts from the donor truck and my old V10 setup i actually made money on the conversion. Only took me a week and a half to do the swap.  However about of month of prep and lining up the parts I needed. Best decision i've made for that truck.  I was only getting 9.5 mpg with the V10 and now i'm close to 19 mpg if I keep my foot out of it but it's hard to do when it runs and sounds so good.

[Mopar1973Man]

Just remember larger wheels and tires will mean less braking performance and shorter brake life span. This will increase wear in the steering and suspension parts on the front axle. That being said I'm a bigger fan of re-gearing the axle than installing larger wheels and tires. 

 

Just consider I've got 301k miles now and only 1 set of ball joints, 1 track bar. Still got factory tie rod ends, steering box and power steering pump. I've only done 1 brake pad change back at 185k so I'm good till about 360k miles yet. Smaller and lighter wheels mean less wear on steering parts and better MPG's being I'm still holding 27.2 MPG as a high mark.

[JAG1]

Nice excerpt about the conversion. How did you know about swapping input shafts? That's pretty cool, I must say.

[martinique78]

11 hours ago, JAG1 said:

Nice excerpt about the conversion. How did you know about swapping input shafts? That's pretty cool, I must say.

Thanks! Lots of reading and knowing that the 2003 and newer dodges came with the same transfer case NP271/273.  NP271 being the manual shift and the NP273 being the electric shift.  Only thing different was the input spline count on the transfer case.