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I need to regear my '08 JKU X and am looking to purchase a new set of gears.
According to the spec chart, even though I'm only running 33" tires, I should still go with 5.86 gears.
This is because I have the 3.8L engine, an automatic transmission, use the Jeep off-highway 95% of the time and mainly climb steep/rough grades at elevations between 9,000 and 13,000ft.
Does anyone know a supplier or source?
Thanks.
 

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5.86 gears will not fit in a stock housing. I believe lowest you can fit is 5.13 non-Rubicon Dana 30 front, 5.38 for a Rubicon dana 44 front.
 

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At one point The was one manufacturer that did produce 5.38 gears for the JKD30 and of course the rear D44 gear set fits both Rubi and none Rubi.

And apparently Revolution still makes them.



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You do not want to put 5.38s in a D30 or even 5.13s for that matter. The pinion gear is tiny and the gear set is weak. I’ve seen grenaded 5.13s in a D30. I wouldn’t run 5.38s in a 44 for the same reason, unless your Jeep never leaves the pavement and you have a light foot.
 

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You do not want to put 5.38s in a D30 or even 5.13s for that matter. The pinion gear is tiny and the gear set is weak. I’ve seen grenaded 5.13s in a D30. I wouldn’t run 5.38s in a 44 for the same reason, unless your Jeep never leaves the pavement and you have a light foot.
Heard this for 5.38 and D44 and the I could only find one instance of a gear failure, and they were running 40s...Not saying it doesnt happen, i just had a hard time finding solid information indicating it is in fact a consistent and know problem.

In the end it didnt stop me from running 5.38s and 37s.:drinks:
 

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something to consider is that while a 3.6 motor makes enough power to perhaps break stuff with 5.13 gears or even 5.38 gears, with a older 3.8 motor the whole reason you want the deeper gears is that it makes 2/3's the power of a 3.6. Less power means that what may be breakage waiting to happen with the 3.6 may be less of an issue with the 3.8. That lack of power driving the desire for deeper gears also means that there is less power that can break stuff.
The reality is, you pay your money and take your chances. If I had a 3.8 motor I would want at least 5.13 gears, and with 37's I would want 5.38 gears.
I am not sure about 5.38 gears with 33's. But I think you would be fine. I think the smaller the tires with 5.38 gears the less chance of breaking something. How streetable it will be, I don't know. But I think if you break something with 5.38 gears and 33" tires it is your fault and not the gear sets.
 

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You do not want to put 5.38s in a D30 or even 5.13s for that matter. The pinion gear is tiny and the gear set is weak. I’ve seen grenaded 5.13s in a D30. I wouldn’t run 5.38s in a 44 for the same reason, unless your Jeep never leaves the pavement and you have a light foot.

I ran 5.38 in D44 with 37" Pitbulls for 3 years 0 issues with gears, but i did snap axles. .. There is more then enough meat there with the Thick Cut JK gears..If you are breaking gears something is wrong..
 

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something to consider is that while a 3.6 motor makes enough power to perhaps break stuff with 5.13 gears or even 5.38 gears, with a older 3.8 motor the whole reason you want the deeper gears is that it makes 2/3's the power of a 3.6. Less power means that what may be breakage waiting to happen with the 3.6 may be less of an issue with the 3.8. That lack of power driving the desire for deeper gears also means that there is less power that can break stuff.
The reality is, you pay your money and take your chances. If I had a 38 motor I would want at least 5.13 gears, and with 37's I would want 5.38 gears.

3.8 does have less HP then the 3.6 at the top and torque is close to same at lower RPM. But the big difference is the transmission. The 3.8 auto requires 4.10 gears just to be in the same ballpark as a 3.6 with 3.21 gears. Think about that you guys who are whinning about your 3.21s on 35s, can you imagine what it would feel like to run the 3.6 with 2.55 gears.
So compare this, the 3.8 with 5.38 gears (2495rpm with 35s @ 70mph) is compared to 3.6 with auto 4.56 gears (2544rpm with 35s @ 70mph).

And people wonder why the 3.8 feels gutless. Drive a 3.8 JKU geared to 5.38 and you'd be surprised..


The reason the OP is looking at a gear chart with 5.86 gears listed is because 5.86 gears (if manufactured) (2718rpm 35s @ 70mph) would be the equivalent to 4.88 gears with the 3.6 (2723rpm 35s @ 70mph)...
 

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You do not want to put 5.38s in a D30 or even 5.13s for that matter. The pinion gear is tiny and the gear set is weak. I’ve seen grenaded 5.13s in a D30. I wouldn’t run 5.38s in a 44 for the same reason, unless your Jeep never leaves the pavement and you have a light foot.
Heard this for 5.38 and D44 and the I could only find one instance of a gear failure, and they were running 40s...Not saying it doesnt happen, i just had a hard time finding solid information indicating it is in fact a consistent and know problem.

In the end it didnt stop me from running 5.38s and 37s.
If you have seen the comparative sizes of the 5.38 pinion and the 5.13 pinion, it’s pretty evident which one will be stronger. But if 5.38s are working for you, you should go with your experience.
 

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You do not want to put 5.38s in a D30 or even 5.13s for that matter. The pinion gear is tiny and the gear set is weak. I’ve seen grenaded 5.13s in a D30. I wouldn’t run 5.38s in a 44 for the same reason, unless your Jeep never leaves the pavement and you have a light foot.
I ran 5.38 in D44 with 37" Pitbulls for 3 years 0 issues with gears, but i did snap axles. .. There is more then enough meat there with the Thick Cut JK gears..If you are breaking gears something is wrong..
It’s a subject on which reasonable minds can differ. I wish I had the 5.38 gearing for my poor old 3.8 pushing 37s, but I personally prefer to have the peace of mind that 5.13s bring. Especially since I’m running a Rubicrawler and occasionally run at 10.8:1 reduction. If 5.38s have been working for you and you’re confident with that set up, great.
 

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I ran 5.38 in D44 with 37" Pitbulls for 3 years 0 issues with gears, but i did snap axles. .. There is more then enough meat there with the Thick Cut JK gears..If you are breaking gears something is wrong..
Only the ring gear is "thick" in the thick cut gears. Its the pinion strength that gets called into question with deeper gears. Im not sure I buy all of it, but there is some truth to it atleast.
 

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Apparently Not true. Check this video out by Nitro Gear. Dispels the myth about the smaller Pinion because it isn’t actually smaller.

.


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Apparently Not true. Check this video out by Nitro Gear. Dispels the myth about the smaller Pinion.

.


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Ah the age of Trump. Science is not science. Physics are not physics. And nothing is true if you believe otherwise. As I said, if 5.38s are working for you, great.
 

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When Spicer/Dana take the 5.38 off the table for the D44 and cites its reasoning as "due to inherent weakness..." I'll start believing the internet myth.


With 8 teeth on all the pinions, but the 5.38 pinion having less of a conical shape to it (less angular force, less chance of spitting the ring, probably why people think it's smaller), and, interfacing with more ring teeth I start to wonder if maybe it isn't actually stronger...
I'd like to see the math, or better yet measured mesh pattern on surface area of teeth in contact between the 5.13 and/vs 5.38.


And for the record, I'm genuinely curious... not trying to stir an internet battle.


Kinda the same thinking that a fine thread bolt is stronger than a coarse thread bolt.
 

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Apparently Not true. Check this video out by Nitro Gear. Dispels the myth about the smaller Pinion because it isn’t actually smaller.

. https://youtu.be/9fAxFNnRpJA


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That video was posted in another forum and this was my response. I wasn't and still am not trying to be argumentative, but rather hoped (and previously failed) to spark a conversation backed by facts on the subject. There is far more to gear design than tooth count...the pinions are not the same.





"I kind of feel like you are feeding the rumor mill here. Most of this info is anecdotal (IMHO). Yes the pinions have the same amount of teeth, but as you say: the pinion is smaller. If it is smaller than chances are the teeth are skinnier and therefore have a smaller cross sectional area. That is what makes them weaker and decreases the shear strength of the tooth.

You also say that they are smaller, but it doesn't make it much weaker. Any time you make it smaller, it is getting weaker. Whether or not it is enough to be negligible, is somewhat a matter of opinion. Some people don't want to sacrifice any strength, whereas others may be willing to take a chance. I would be interested in someone measuring the actual tooth thickness and crunching some numbers to give us an actual tangible strength comparison vs someone saying "its smaller but not that much smaller" (not a direct quote).

I think a more solid reason for backing up the JK 5.13s and 5.38 is actually the differences in the JK rear next gen 44 vs past typical 44s. The ring gear is 8.9" (8.8 depending on who you ask) whereas past 44s have a 8.5". The gear ratio can be figured out by the ratio of the pitch diameters (the effective diameter of the gear where the teeth meet). That means that if you increase the ring gear diameter, you must increase the pinion diameter as well to maintain the gear ratio. So the rear JK pinion should also be larger for a given gear ratio. So while a JK 5.13 and 5.38 may have a smaller weaker pinion than a JK 4.88, the JK versions are bigger than past iterations. It would be really interesting to see a JK 4.88 and 5.13 pinion next to an old school 4.88 and 5.13.

I don't necessarily disagree with what was stated in the video, just how it was presented. I personally am planning on regearing my Rubi axles to 5.13s for 37-38s in the semi-near future."
 

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Good discussion.. As stated there are no glaring facts that state 5.38 gears in the JKD44 are an accident waiting to happen. all I can say is I ran them for 3 years over 100,000klm on and off road with no problems.. Even broke front axle shafts but the gears are tough and were still quiet when I trade the Jeep in 2 years ago. Its still on the road up north.

I like that analogy of the Fine thread opposed to coarse thread strength, I too believe that comes into play..
 

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I too find a problem with the "its smaller" argument. Ok, maybe so, but that tells me very little about strength, someone needs to quantify the strength differences for it to actually be useful.

Since this information doesnt seem to be available, i went with what i thought was the next best method to determine if i would see issues running 5.38s. I searched high and low on forums and asked questions for first hand experience about gear failures. At the end of the day I found one issue with 5.38s actually failing, on a rig running 40s on stock D44s. I also spoke to my gear installer, who i trust, and asked him. To me, the major complaints from 5.38s seem to stem from the high RPMs on the highway. I can live with that.

So 5.38s, 3.6L, auto trans and 37s is how I am currently running with no issues so far.
 

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I always wonder why we dont see any actual destructive test for the R&Ps. I feel like a manufacturer could spare a set of a few different ratios for the name of science....
 
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