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Discussion Starter · #1 ·
So who on here is running the manual 3.69 diffs in an auto? I am not interested in titan swapping and know that the 3.69 is the lowest gear ratio that I can get for the R180. Just curious to know who is running this setup, and what issues have you had. My biggest concern is the lack of 6th gear overdrive for the highway speeds and running at too high of an RPM.

FYI this is a daily driver and will continue to be a daily driver. Just weighing options for future build ideas. Thanks.
 

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What size tires?

Very few if any are running that ratio. I looked into a while back.

I think with 35's 3:69 Or 3.56 for 33's....would be good for a daily. While getting some power back.
This! I know of at least 2 rigs runing 35s and 3.69. Tires size is going to play a big part of that.
 

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Discussion Starter · #4 ·
I am running 33's. Not going to go larger than that. Looking to gain some torque and low speed crawling ability. Making it easier to stay off the skinny pedal. What RPM would I be running at at 65-70 with the 3.69's compared to the 3.36's?
 

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I think Penski61 & Sexy6Chick had that ratio in their 2nd gen before Penski SAS'd it ... it's not a very popular ratio for some reason (probably due to the rarity of used diffs, as compared to 3.36 diffs ... especially for the rear), but I think it's a great upgrade option for AT guys.

Here's a ratio comparison (this is for 2008 - my year - but shouldn't be different for other years):
---- - - MT - - - AT
1st - 4.368 - 3.842
2nd - 2.518 - 2.353
3rd - 1.743 - 1.529
4th - 1.283 - 1.000
5th - 1.000 - 0.839
6th - 0.769 - N/A

... so you can see that it's not so much that you're not going to have OD, 'cause you still do, but that you just don't have the same choices (ratio spread) as you go up to your 1:1 gear ... at least by comparison to the MT.

You're an S model, so you have 3.13 gears if I'm not mistaken, that'll make the 3.69 gears ~18% deeper than stock ... actually a bigger step than going to 4.10s from 3.69 (~11%), but not quite as big as going all the way to 4.56 (~23.5%) ... if you used 3.54 instead, it'd be more like a 13% jump. So that's the difference you can expect in RPM, if you're on the same size tire ... going to bigger tires, as well, will bring that percentage back closer to 0, by the percentage increase in size.
 

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You can figure out your RPM by using the following:

ground speed = (tire-loaded-circumference) * wheel-rpm
wheel-rpm = engine-rpm / (axle-ratio * trans-ratio)
tire-loaded-circumference => either directly measured from the ground, by marking a single tire revolution ... or, measure ground-hub distance (loaded radius) and compute circumference = (2 * pi * loaded radius)
pi = 3.14159 ... and just make sure you have all the unit conversions correct (you'll get ground speed in the same units as circumference, for example ... and you'll need to get a conversion to the right time units in there if you're looking for ground speed in mph).

At one point I put together a spreadsheet that computes all of this ... PM me your e-mail address, I'll send it to you and you can play around.
 

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Yea um, parenthesis and pi, no thanks there is a link to diff wizard above lol!!!!
... okay ... but that assumes you want the engine RPMS for the trans ratios they're calculating for ... which aren't our trans ratios. They calculate for trans ratios of: D in an AOD, 0.7 OD, 1:1, and 0.8 OD ... close to ones that we have, but not exact ... and TBH, I have no idea what ratio D is in an AOD. I guess the 1:1 ratio is helpful, though.
 

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This chart might be helpful. It's built around Jeep gear ratios, but you can get a pretty good idea from it what your RPMs are going to be in the scenarios you are asking about.

The chart is based on 1:1 transmission (so for you, 4th gear) and 65mph.

So, with 3.13 gears and running 33's, you should be at about 2080 rpm in 4th gear. If you go to 3.69 gears and stay with 33's, same speed and gear will have you at about 2440 rpm. Moving up to 35's with 3.69 puts your rpms at about 2300.
 

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Discussion Starter · #11 ·
Thanks everyone! Lots of great info.
 

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^^^ ... it also assumes (with the power/efficiency comments ... the red/yellow/green labeling) the Jeep 4L (presumption ... really it's whatever baseline motor they're looking at) power band and efficiency curves. Ours for the VQ40 are pretty different ... so keep that in mind. But, yes, the RPM numbers should be correct for a 1:1 ratio, assuming their math is good (probably an OK assumption).
 

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2050 would be your RPM's at 65mph with 3.69 gears and the final overdrive ratio of .839..

This calc lets you plug whatever you want in and spits out accurate info. I have an excel spreadsheet with all these formulas built in somewhere that I verified this calculator against.

http://www.powerblocktv.com/resources/

The cool thing about this is that you can incorporate your transfer case as well so you will know what your RPMs will be while locked into 4LO
 

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The other thing to keep in mind, if you're keeping your r180, is the carrier split. Since you're coming from the 3.13 ratio, you can go to 3.54s in your diff but for 3.69s you'd either need the whole carrier from a donor used diff (in which case you might as well just put the whole thing in), a new 3.69 & up ARB, or you'd have to do some figuring on what changes you'd need to make ... it'll either be a bit of machining on the ring gear or a spacer between the ring and carrier, depending on which way the ring-to-pinion offset moves for the split.
 

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They're obviously not on the site (I'm sure you're just as capable as me, at reading what's listed for sale) ... but I'm not sure why. I have an e-mail in to Steve for more info.

Have you seen anyone who has installed them? I haven't heard of it yet ... maybe there's a reason? When did they go up on the 4x4 Parts site?
 

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Discussion Starter · #20 ·
I am probably going to go with a donor diff to save costs. Install costs on gears is a bit too much. Just weighing my options right now as far as gearing goes.
 
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