What is needed to upgrade 3.21 to 3.73 gears? - Jeep Wrangler Forum
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Old 08-18-2015, 11:52 AM
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Join Date: Aug 2015
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What is needed to upgrade 3.21 to 3.73 gears?

I am looking to upgrade to a lower gear ratio. I accidentally ordered my '15 JKU with a 3.21. I plan on upgrading to a 2.5 lift with 33s. So naturally I have to upgrade to 3.73. I don't need 4.10s as I am a daily commuter so input on that please. What is needed to go to a 3.73?
- I assume I need to order 2, for the front and rear
- Planning on slighty used or new takeoffs
- Does it matter if they are from or for a GMC, Mustange, etc (eBay)?

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Old 08-18-2015, 12:07 PM   #2
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Location: Littleton, CO
Posts: 3,428
Honestly, I'll recommend upgrading to 4.10's for 33's. The overall performance will be better and you won't see any mileage decrease. But ultimately, it's your choice.

One option that may save you money is to surf craigslist to see if you can find someone selling stock axles with 3.73's already in them. It would be a bolt-in swap to change them over and they will most likely cost less than regearing.

I would stay away from used gears. No installer I know would warranty their work with used gears. The chances of getting them installed wrong is too great.

Also, gears are very specific to each axle. You can't use GM or Ford parts in the Dana axles under a Jeep. They have to be gears specifically for a front JK Dana 30 and a rear JK Dana 44. There's no way to make any other gears fit.

Beyond that, the installer you choose will be able to order all the right parts for you. But so you know that you're not getting ripped off, these parts will be in the list:

- 3.73 gear set for both the front and the back.
- master install kits for both the front and back. These have the bearings and shims needed to set up the gears correctly in the axle.
- One new differential carrier for the front axle. Going from 3.21 to any other gear (3.73 to 5.13) in the front axle requires a new carrier. You have the option to upgrade from an open differential to a locker of some kind and only have to pay the difference in price. Read up here if you don't know much about lockers: The Basics of Differentials and Lockers
- Gear oil

That's all that is absolutely necessary.

The rear axle uses one carrier for all gear ratios so you don't need to upgrade it if you don't want to. But it makes sense to do so when you do gears because you are already doing the work and you only have to pay for the part. There is no additional labor (aside from running the control for a selectable locker) if you upgrade while you're in there. Even if you're not going to go off road a lot, upgrading to a limited slip in the rear makes sense for on-road driving if you live where you get ice and/or snow.

2001 XJ - 4.0/AW4/NP242/D44/D44
1991 MJ - 2.5/AX5/D35
1987 MJ - 4.0/BA10-5/D35
1986 J10 - 401/700R4/NP208/D44HD/D60FF

My gear ratio calculator
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Old 08-18-2015, 12:08 PM   #3
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Posts: 1,542
The 4.10s would be a better fit for the 33s. You cannot use gears from other vehicles. If you were to find used gears for your exact vehicle it is not worth it. Most shops will not install or if they do they will not warranty them. This is not a area where you can save a few dollars. It's one of those do it right or not at all kind of things. The labor is the expensive part.
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Old 08-18-2015, 12:09 PM   #4
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Guess we hit send at the same time.
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Old 08-18-2015, 03:38 PM   #5
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Join Date: May 2014
Location: Connecticut
Posts: 132
^^^ all good information above. Definitely need bigger than 3.73 - check out this chart for recommended gear ratios...
Jeep Parts, Jeep Accessories & Jeep Soft Tops From The Jeep Parts Experts - Quadratec
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Old 08-18-2015, 05:28 PM   #6
Join Date: Aug 2013
Location: North GA
Posts: 615
Trade it for a Rubicon. You'll spend about the same or less than trying to modify what you have and get some other goodies to boot.
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Old 08-21-2015, 12:25 AM   #7
Join Date: Sep 2012
Posts: 23
Did you already take possession of the Jeep? I ordered one with 3.21s and went back a couple days later and told them I wanted to order a second one with 3.73s and limited slip. I told them to sell the first one with 3.21s on the lot. They did it for me.

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