Gear Ratio for 35's

SuspectLJ

SuspectLJ

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Hey everyone, i got a 2004 LJ. 4.0 with the automatic transmission.

I've been slowing buying parts for my build and reading previous posts on the forum as a guide. I'm now looking to buy the right gears for 35 inch tires.

From what i understand, the recommendation here is to go with 5.13 and 5.38's if you have Rubi axles.

I went and spoke with a shop near me where i was planning on having the rear gear done and they swearing up and down that 5.38 gears is way overkill for 35's and the pinion size will be way to small.

they're recommending i go with 4.88s.

Are these dudes just flat out wrong or are 5.38s no longer recommended?

thanks in advance.
 
All going to depend on if you’re going for primarily a crawler or DD. Lower is going to be better for crawling obviously.
Also would recommend plugging some numbers into this gear ratio calculator to help you determine your ideal gear ratio according to the RPMs you want to run at different speeds/in different gears.
Grimm Jeeper Gear Ratio Calculator
 
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SuspectLJ said:
Hey everyone, i got a 2004 LJ. 4.0 with the automatic transmission.

I've been slowing buying parts for my build and reading previous posts on the forum as a guide. I'm now looking to buy the right gears for 35 inch tires.

From what i understand, the recommendation here is to go with 5.13 and 5.38's if you have Rubi axles.

I went and spoke with a shop near me where i was planning on having the rear gear done and they swearing up and down that 5.38 gears is way overkill for 35's and the pinion size will be way to small.

they're recommending i go with 4.88s.

Are these dudes just flat out wrong or are 5.38s no longer recommended?

thanks in advance.
Click to expand...

Lies, that shop doesn't know what they're talking about. I'm running 5.89s on my 04 auto LJ, (which are 2 steps below 5.38s, ie the equivalent of going (4.88>5.38), and they are still not low enough.

My highway rpms are still less than in a bone stock 97-02 TJ with the 32RH.

Go 5.38s if you have a front Dana 44 and be done with it.
 
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SuspectLJ said:
Hey everyone, i got a 2004 LJ. 4.0 with the automatic transmission.

I've been slowing buying parts for my build and reading previous posts on the forum as a guide. I'm now looking to buy the right gears for 35 inch tires.

From what i understand, the recommendation here is to go with 5.13 and 5.38's if you have Rubi axles.

I went and spoke with a shop near me where i was planning on having the rear gear done and they swearing up and down that 5.38 gears is way overkill for 35's and the pinion size will be way to small.

they're recommending i go with 4.88s.

Are these dudes just flat out wrong or are 5.38s no longer recommended?

thanks in advance.
Click to expand...

Below are calculations for a typical 35" tire with both 513 and 538. Is 2762rpm at 75mph a concern with this shop? Personally, I don't think it is enough rpm at that speed.

42rle/33.875"/5.13=2633rpm @ 75mph

42rle/33.875"/5.38=2762rpm @ 75mph
 
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James9100 said:
All going to depend on if you’re going for primarily a crawler or DD. Lower is going to be better for crawling obviously.
Also would recommend plugging some numbers into this gear ratio calculator to help you determine your ideal gear ratio according to the RPMs you want to run at different speeds/in different gears.
Grimm Jeeper Gear Ratio Calculator
Click to expand...

It makes no difference if this is a daily driver or a rock crawler. Both benefit from lower axle gearing and we already cannot gear deep enough for the highway without encroaching deep into significant compromises, both monetarily and mechanical.
 
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@PNW_LJ @jjvw @James9100 @GPK03X
Thanks for the quick replies guys i appreciate it. I had a feeling they were mistaken but i figured i would double check here as this is my first time "building" (buying bolt on parts haha) a tj/lj.

I have a front 44 out of a TJR I'm swapping in and I'm gonna keep the stock 44 in the rear.

https://www.polyperformance.com/rev...d-rear-gear-package-for-03-05-jeep-tj-rubicon

This kit should work fine even though my rear axle isn't a "rubicon" 44 correct?

incase anybody else is in the market, with code "poly10" this is the lowest price i've found for a set of revolution gears. ($610)
 
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SuspectLJ said:
@PNW_LJ @jjvw @James9100 @GPK03X
Thanks for the quick replies guys i appreciate it. I had a feeling they were mistaken but i figured i would double check here as this is my first time "building" (buying bolt on parts haha) a tj/lj.

I have a front 44 out of a TJR I'm swapping in and I'm gonna keep the stock 44 in the rear.

https://www.polyperformance.com/rev...d-rear-gear-package-for-03-05-jeep-tj-rubicon

This kit should work fine even though my rear axle isn't a "rubicon" 44 correct?

incase anybody else is in the market, with code "poly10" this is the lowest price i've found for a set of revolution gears. ($610)
Click to expand...

I haven't looked at the link.

If you are using factory Rubicon lockers, then you need thick cut gears for anything deeper than 3.73, which includes 5.38.

For other carriers/lockers, you need to pay attention to the carrier break to determine if you need standard cut or thick cut gears.
 
jjvw said:
You're the guinea pig. We might find out why 5.89 in the Dana 44 isn't something we see in the wild even though we know how to get them in there.
Click to expand...

You're right, we will see.

Cost is a bland reason. There are many people out there that buy complete Rubi axles, only to strip them and slap in new shafts, lockers (especially that 35 spline BS), and the wrong gears. Hopefully my experience will be a positive one, to help those people add the best gears to their shopping carts.
 
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jjvw said:
I haven't looked at the link.

If you are using factory Rubicon lockers, then you need thick cut gears for anything deeper than 3.73, which includes 5.38.

For other carriers/lockers, you need to pay attention to the carrier break to determine if you need standard cut or thick cut gears.
Click to expand...

i won't be reusing the factory lockers and will most likely go with an Eaton e-locker
 
jjvw said:
It makes no difference if this is a daily driver or a rock crawler. Both benefit from lower axle gearing and we already cannot gear deep enough for the highway without encroaching deep into significant compromises, both monetarily and mechanical.
Click to expand...

I think it does depend. Personally just from a quick look at the grimm jeeper calculator I’d prob go with 5.13 over 5.38.
5.13=2400rpm @ 70mph
5.38=2517rpm @ 70 mph
Keep in mind these are rough numbers, I just used 34.8” for the tire size.
I tend to like the 2300-2500rpm range, although I’m sure someone will tell me that’s lugging the 4.0L. For cruising on the highway for longer periods I’d rather be closer to 2400.
I also don’t have any crawling around me to do and live on the FLATEST prairies of Saskatchewan, if I did crawling I would prob do 5.38.

Disclaimer: I’m NOT a gearing expert. I’m still running 3.07 with 33s and an NV3550 which SUCKS on all levels.
 
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PNW_LJ said:
You're right, we will see.

Cost is a bland reason. There are many people out there that buy complete Rubi axles, only to strip them and slap in new shafts, lockers (especially that 35 spline BS), and the wrong gears. Hopefully my experience will be a positive one, to help those people add the best gears to their shopping carts.
Click to expand...

Despite appearances my build is fairly conservative and low risk. I had the opportunity to do 589, but chose not to specifically because of the known issue with driveshaft rpms and harmonic vibrations already occurring frequently with 538 when added to the 241 and 42rle.
 
jjvw said:
Despite appearances my build is fairly conservative and low risk. I had the opportunity to do 589, but chose not to specifically because of the known issue with driveshaft rpms and harmonic vibrations already occurring frequently with 538 when added to the 241 and 42rle.
Click to expand...

Vibes can and do happen with 5.13s and even 4.88s
 
SuspectLJ said:
Hey everyone, i got a 2004 LJ. 4.0 with the automatic transmission.

I've been slowing buying parts for my build and reading previous posts on the forum as a guide. I'm now looking to buy the right gears for 35 inch tires.

From what i understand, the recommendation here is to go with 5.13 and 5.38's if you have Rubi axles.

I went and spoke with a shop near me where i was planning on having the rear gear done and they swearing up and down that 5.38 gears is way overkill for 35's and the pinion size will be way to small.

they're recommending i go with 4.88s.

Are these dudes just flat out wrong or are 5.38s no longer recommended?

thanks in advance.
Click to expand...

5.38’s are not low enough.
 
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James9100 said:
I think it does depend. Personally just from a quick look at the grimm jeeper calculator I’d prob go with 5.13 over 5.38.
5.13=2400rpm @ 70mph
5.38=2517rpm @ 70 mph
Keep in mind these are rough numbers, I just used 34.8” for the tire size.
I tend to like the 2300-2500rpm range, although I’m sure someone will tell me that’s lugging the 4.0L. For cruising on the highway for longer periods I’d rather be closer to 2400.
I also don’t have any crawling around me to do and live on the FLATEST prairies of Saskatchewan, if I did crawling I would prob do 5.38.

Disclaimer: I’m NOT a gearing expert. I’m still running 3.07 with 33s and an NV3550 which SUCKS on all levels.
Click to expand...

I have been doing this for a while. The Jeep was my daily for a long time and the best gearing it ever had was when it was 3k at 75mph. And that is still a lower rpm than what the factory rolled out in the early TJs with the 32rh. Almost everyone out there is not regearing deep enough.
 
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