Alternate Gear Ratio Idea... Maybe... 3.92???

Cobalt-60 · **Posted:** Thu Feb 03, 2011 10:49 pm

Don't know if anyone has discussed this as an alternative or not but I was looking around online this evening (more just kind of seeing what the options were...) and noticed that there are ring & pinion sets available for the Chrysler (Corporate) 8.25" in 3.55 (which the 6-spd manual has), 3.73 (which most of the A/Ts have), 4.10 (which the 2.5L had), and 3.92... :shock:

I did some checking around and it "appears" that the 3.92 gears will indeed fit in the Liberty's rear diff. "Okay, I thought, so what...? Chances are that there are no 3.92 gears made for the Dana 30a, right?"

But then I kept on checking and ran into references for 3.92 gears for the "186FIA", which on further investigation I learned is the "186 Front Independent Aluminum" differential (AKA "the 27 spline Dana 30a")...

So, if it is possible to install a set of 3.92 gears in the KJ/KKs, what would be the benefit of doing such compared to going to 4.10s from 3.73s or 3.55s? Can anyone think of a situation where this might actually make sense? Yes, this is more of an intellectual exercise than anything else, but I was wondering if the combination of a 3.7L mated to a 42RLE (AKA "minivan trannie") with 3.92 gears might very well be in the sweet-spot between mileage and power-loss restoration especially after moderately larger tires (245/75-16 ATs), lift (2.5"), heavier armor (Skid Row & Rusty's), and all steel rims.

Thoughts?

[NOTE: it turns out that the 3.92s are a pretty common option with the 8.25" in the Dodge Dakota and is also an option in the "KA" AKA Dodge Nitro]

waywardtravel · **Posted:** Thu Feb 03, 2011 11:21 pm

Very interesting. If I remember right when I ran the numbers (many calculaters) going from the factory tires to the 245/75/16's, the effective gear ratio to come up with the more or less exact same rpm at 65mph was 3.92:1. That would mean theoretically the exact same fuel consumption other than the fact that the weight of the tires changed and the rolling resistance changed, but I personally have no idea why anyone would want to do that over gaining some power in a Jeep.
Maybe if I had a 6speed but I'd personally still want 4.10's. ow I never thought of a six speed with those gears because I didn't know they used 3.55's

Cobalt-60 · **Posted:** Thu Feb 03, 2011 11:23 pm

Tom, that is kind of what I thought. However, 3.92 actually WAS listed as a gear option for the 186FIA in the 2002 KJ FSM (page 33):

http://www.scribd.com/doc/3287027/Jeep-KJ-2002-Liberty-Service-Manual-PDF-Secc-03-Differential-and-Driveline

:shock:

tommudd · **Posted:** Fri Feb 04, 2011 9:10 am

With a lifted KJ, extra weight of tires and wheels, skids and everything else we throw on I can see no use of going to the 3.92s.
With paper and pencil it may look as if that would work best but in real world driving I would still go to 4.10s
Not that much difference ( between 3.92 and 4.10s)
Some have even told me I was foolish even to go from 3.73s to 4.10s since there wasn't much difference in their eyes :ROTFL:

foolish people :goink:

Cobalt-60 · **Posted:** Fri Feb 04, 2011 11:01 am

Yeah, on additional checking it appears that this is "possible" and "might" have been a potential consideration when the KJ was first put out (we know about all the changes that happen between 2002 and 2003) but that I have not been able to find any case where there are 3.92 ring & pinion sold for the Dana 30a/186FIA, so it is kind of a moot point. But if somebody REALLY wants to they can go the other way... there are 3.21 gears sold for the KA (Dodge Nitro)... :lol:

It does bring up another question though -- has the Dana 30a/186FIA ever been used in any other vehicle? Or are we just the "lucky" few... :wink:

Alternate Gear Ratio Idea... Maybe... 3.92???

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