Re-Gearing a JLU

Carlton_Banks

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When adding larger it is important to regear to regain the correct power band/leverage for each gear. With the JK we had 5 gears on the automatic. With gearing very low you would gain the leverage needed to get the tires moving in 1,2,3 gear but would run semi high RPMs at highway speeds. There was a little give and take but was ok.

With the JLU and an 8 speed tranny, I am assuming we will not need to gear as low/gearing low would almost cause us to lose 1/2/3 gears. Does anyone have any insight on this? What gears would be appropriate for what tire size? What would RPMs be at 70 witch common combinations?





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AZCrawl

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I don't think you have to go as low as you would on the JK...you could probably do 4.56 on 37's.
 
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Carlton_Banks

Carlton_Banks

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I usually run 38-40 inch tires with 5.13-5.38 gears. This would make gears 1-3 almost none existent I would think. I guess only time and experimentation will tell.
 

Jeepsterfreak

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Yes, check out Northridge4x4's website.
Cool. Thanks.

One question about the rear LSD option. Do we know what fits the LSD in the D44? I assume these Spicer 200mm rear gears are only for the standard D35 open diff and not compatible with the LSD.

I'm guessing the LSD ring gear is larger than 200mm since it requires the D44 axle upgrade but not compatible with Rubi ring/pinion.
 
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matto

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Cool. Thanks.

One question about the rear LSD option. Do we know what fits the LSD in the D44? I assume these Spicer 200mm rear gears are only for the standard D35 open diff and not compatible with the LSD.

I'm guessing the LSD ring gear is larger than 200mm since it requires the D44 axle upgrade but not compatible with Rubi ring/pinion.
not sure only know they have the 220 and 210 along with the sport versions.
 

AZCrawl

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So, I found this website for gear calculations...

http://www.grimmjeeper.com/gears.html

According to this site, for a JLUR or JLR, the sweet spot for gearing for 37 inch tires is 4.56.

It is so sweet in fact, that it will increase your crawl ratio from 77.2 to 85.9 and on the top end you will run fewer RPM's at highway speeds!

I plugged in all of the automatic transmission gear ratios with the 4.10 transfer case. Going to 4.88's increases your RPM's at highway speeds.

So I am definitely going to re-gear with 4.56's.
 

matto

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So, I found this website for gear calculations...

http://www.grimmjeeper.com/gears.html

According to this site, for a JLUR or JLR, the sweet spot for gearing for 37 inch tires is 4.56.

It is so sweet in fact, that it will increase your crawl ratio from 77.2 to 85.9 and on the top end you will run fewer RPM's at highway speeds!

I plugged in all of the automatic transmission gear ratios with the 4.10 transfer case. Going to 4.88's increases your RPM's at highway speeds.

So I am definitely going to re-gear with 4.56's.
Nice! I guessedamated correctly for running 38s or 40s
 

matto

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So, I found this website for gear calculations...

http://www.grimmjeeper.com/gears.html

According to this site, for a JLUR or JLR, the sweet spot for gearing for 37 inch tires is 4.56.

It is so sweet in fact, that it will increase your crawl ratio from 77.2 to 85.9 and on the top end you will run fewer RPM's at highway speeds!

I plugged in all of the automatic transmission gear ratios with the 4.10 transfer case. Going to 4.88's increases your RPM's at highway speeds.

So I am definitely going to re-gear with 4.56's.

Where did you get your numbers for each gear? can you send a screen shot of what it looks like?
 

AZCrawl

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Where did you get your numbers for each gear? can you send a screen shot of what it looks like?
From these forums, you should be able to find them via search, but here they are.

  • Gear Ratios:
    • 1st - 4.71
    • 2nd - 3.13
    • 3rd - 2.10
    • 4th - 1.67
    • 5th - 1.28
    • 6th - 1.00
    • 7th - 0.84
    • 8th - 0.67
    • Reverse - 3.53
 

offcamber

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This is for JL? There are aftermarket options available for the new axle already?
There have been gears for them for a while. It's the same axles used in other vehicles (Chevy Colorado for one).
 

utefan

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If I understand my friend who has been in the drive line business for years correctly, to establish a gear ratio close to factory specs a rather simple formula is applied. Determine the size of tires you want to run, multiply that tire diameter by the gear ratio currently in your vehicle then divide that total by the diameter of the tire the factory provided.

Perhaps an example will provide clarity. I purchased a 2018 JLU Rubicon it came with 33" tires and has a 4.10 gear ratio. Suppose I would like to run 37" tires. The calculation to get back to factory specifications is 37 (the tire size I want to run) multiplied by 4.10 (the gear ratio currently in the vehicle) = 151.7 divided by 33 (the diameter of tires the factory provided) = 4.5969697 Now compare the results of our formula to the available gear sets and choose the gear set that most closely matches, in our case it would be 4.56

I hope this is helpful and sheds a little light on the subject. Disclaimer: I am not an expert, nor do I play one on TV. Seek advise from real experts before selecting your gear set.... or use the calculation above at your own risk.

Happy Jeeping.

Cheers!
 

intentsrig

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Cool. Thanks.

One question about the rear LSD option. Do we know what fits the LSD in the D44? I assume these Spicer 200mm rear gears are only for the standard D35 open diff and not compatible with the LSD.

I'm guessing the LSD ring gear is larger than 200mm since it requires the D44 axle upgrade but not compatible with Rubi ring/pinion.
The Rubicon 220mm ring gear and the LSD 44 on the sports etc are the same 220mm ring gear. The only difference is the tube length is wider on the Rubicon and the locker of course. Both are 32 spline now as well.

Northridge’s website does not allow you to select 220mm if you have a sport so you’ll have to change your vehicle. They told me awhile back they were going to fix it.

I’m going 4.88’s with my 35’s. I have a manual and want to get my crawl ratio as low as possible. 4.88’s will take me from 48:1 to 68:1. I miss my Rubicon t-case!
 

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