Why should I regear?

[Bobsled]

Regearing your Jeep is one of the best things that you can do to your Jeep.
It is one modification that you will literally feel every time you press the accelerator pedal.

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[Old Jeeper]

So I know there are several threads on regearing, but they mainly centered on what ratio to go with. I want to know the why’s. 2018 JLU Sport S, 3.6, auto, 35x12.5x17 MT Baja Boss AT, 2.5-3” lift(already installed when purchased used).

Why should I spend ~$3000 to regear just because I have 35s and a lift?
Is it just about seeing 8th gear again?
mpg improvement? How much?

Or is there a mechanical/performance reason?

I am not a mechanic nor pretend to be one, so please don’t get to technical.

MAY BE?

OldJeeper Theory of Jeep Relativity

Gears multiply TORQUE. Do you NEED that final drive ratio?

Rock Crawlers Index (RCI)

Base Metric: Jeep TJ Rubicon Stick sift:

1st gear: 4:1
Transfer case: 4:1
Axle ratio: 4.1:1

Multiply = 65.6

Divide by tire height

1.87 = Performance Metric for Rubicon

Take the formula and input your numbers and the result will be mechanically relative to the Rubicon.

This will tell you how each component affects outcome and performance Vs a known index.


FAQ:

Yes you can use any Jeep model, or year as your index. I use the Rubicon because I have had a TJR, JKR, and JLR.

Why do you use the height of the tire: The final Ratio as commonly addressed is nothing but Transmission, Transfer case, and axle ratio. Test my theory: Put on a set of OEM 32s and head out to Hell's Revenge, then put on a set of 37s or 40s...

 

[bcupton]

Changing the diameter of the tire without compensating with a gear change can affect engine performance and potentially can increase transmission wear.

Also, it improves off road performance.

Some more explanation here:

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

^
Totally agree! It took me 2 years to finally spend the money on regearing (JLUS 3.6, auto) after I installed a lift and 35s. BEST Jeep decision ever was to regear. You will stop the wear and tear on your tranny. I went from 3:73 to 4:56.

 

[Steph1]

Simple. The satisfaction of having your Jeep turn 35s as effortlessly as it did the stock tires on stock gearing is an experience you have to live to understand. That’s what I achieved with 315/70r17 on 4.56 gears on my 2018 3.6 with auto tranny.

 

[BDinTX]

If I were you I'd look for a set of Rubicon take off axles (preferably xr but even the 4.10s will be good for 35s). They can usually be had for about the same price and then you gain stronger/ wider axles, lockers, and gears. If you get a set from a diesel or xr you gain steel knuckles too.

Unfortunately the diesel doesn't come with the steel knuckles, I've checked a few times and the magnets still won't stick. They all come with 3.73 gears too so I don't think they'd check off enough boxes to be worth it. Agree the XR axles would be the way to go if you can get 'em cheap.

 

[grimmjeeper]

Unfortunately the diesel doesn't come with the steel knuckles, I've checked a few times and the magnets still won't stick. They all come with 3.73 gears too so I don't think they'd check off enough boxes to be worth it. Agree the XR axles would be the way to go if you can get 'em cheap.

Yep. My knuckles on the diesel Rubicon were aluminum. Swapped them to Reid when I did the truss and ball joints.

 

[CarbonSteel]

I was surprised when I went from 33s to 37s. I assumed my shop would insist on regearing since I have 3.73s. They never mentioned it during my build and near the end of installation, I asked them. They said to just leave it and see what I think for awhile and that it may be just fine.

The day after it was completed, I had a 6 hour drive, wheeled for 5 days including 4 BOH trails and the 6 hours back home. It pretty much seemed perfect.

3.73 = 3.0TD engine?

If so, no need to re-gear. The diesel makes enough torque to compensate. A 3.6L on the other hand will not...

 

[grimmjeeper]

3.73 = 3.0TD engine?

If so, no need to re-gear. The diesel makes enough torque to compensate. A 3.6L on the other hand will not...

Yep. I'm running 3.73s with my diesel and 37s. It works well enough.

I think 4.10 would be better. But not enough to justify doing the swap.

For the 3.6 I would do 4.88 as a minimum for 37s. And likely want to do 5.13 or 5.38.

 

[CarbonSteel]

Yep. I'm running 3.73s with my diesel and 37s. It works well enough.

I think 4.10 would be better. But not enough to justify doing the swap.

For the 3.6 I would do 4.88 as a minimum for 37s. And likely want to do 5.13 or 5.38.

Fully agree...

 

[dpike]

4.88 with 35s on the 3.6 woke my jeep up. I regret nothing about every dollar spent on regearing.

 

[BDinTX]

@CarbonSteel based on the gold rear tow hook and exhaust I think @ParadigmDawg has a 392 which I believe also had 3.73 gears in non-XR form.

 

[WannFly]

Changing the diameter of the tire without compensating with a gear change can affect engine performance and potentially can increase transmission wear.

Also, it improves off road performance.

Some more explanation here:

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

BEST explanation I have ever read on this topic. 4.88 for me then, gotto get the 37s first though

 

[CarbonSteel]

@CarbonSteel based on the gold rear tow hook and exhaust I think @ParadigmDawg has a 392 which I believe also had 3.73 gears in non-XR form.

Oh...pfft...then even less reason to regear

 

[grimmjeeper]

Oh...pfft...then even less reason to regear

That does change the math a little. Especially with the 2.72:1 transfer case.

I maybe would go to 4.56 with a 392 and 37s if I regeared. No need for really short gears like you need with the 3.6. 4.10's may work for the 392 but again, not enough to swap for 3.73s.

 

[ParadigmDawg]

@CarbonSteel based on the gold rear tow hook and exhaust I think @ParadigmDawg has a 392 which I believe also had 3.73 gears in non-XR form.

Correct

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