Re-gearing your Jeep?????

Are you aware of what the consequences are? Do you know what is the performance change involved?

Here is my 2 fils worth homework on re-gearing and observations on the current/standard JK Jeep. I will be repeating the test after re-gearing to share with you the information and hopefully help some of you to decide whether to re-gear or not.


Background:

Differential Gear ratio or the R&P ratio is the ratio of your drive shaft revs to the revs of your tires!!!! Example is 3.92 means the drive shaft turns 3.92 times to make the wheels turn once. Hence 4.56 means more drive shaft turns i.e. more engine revs…. Hence the power gain and speed loss on the same tire size.

So, if you upgrade the tires to bigger size (provided you worked out the clearance/lift) re-gearing to higher ratio brings you back to the power band you were on stock. The table of R&P gear ratio against tire size is available on many places to look at.

T-Case gear ratio: is the ratio of your gear box revs to the revs of the drive shaft(s). Example is 2.72:1 means your gearbox turns 2.72 times to make the drive shafts turn once, hence the low gear ratio 4.0:1 means the gearbox turns 4 times to make the drive shafts turn once hence more torque (rock crawling/towing power)

Additional information and specs of the Gearbox ratio and tires is taken from Jeep specs manual and added to the end of this post for your easy reference.

The test done is recording the speed (taking GPS speed for best possible accuracy) at various gearbox speeds and fixed engine revs, Test was taken while driving 2-Hi, 4-Hi and 4-Low T-case positions.


Current/stock setup:

Manual gear 6 gear
Axle R&P (Ring and Pinion) ratio is 4.1
T-Case 4-Hi does not affect the gear ratio, but rather links both ends (front and rear) to the gear box.
T-Case 4-Low ratio is 4:1
Stock tires as per the table at the end of this post.


The Test:

1- Driving on 2-Hi

6th Gear, Engine revs 3, speed 130km/h
6th Gear, Engine revs 2.5, speed 110km/h
6th Gear, Engine revs 2, speed 90km/h

5th Gear, Engine revs 3, speed 108km/h
5th Gear, Engine revs 2.5, speed 90km/h
5th Gear, Engine revs 2, speed 75km/h

4th Gear, Engine revs 3, speed 86km/h
4th Gear, Engine revs 2.5, speed 73km/h
4th Gear, Engine revs 2, speed 60km/h

3rd Gear, Engine revs 3, speed 63km/h
3rd Gear, Engine revs 2.5, speed 53km/h
3rd Gear, Engine revs 2, speed 42km/h

2nd Gear, Engine revs 3, speed 42km/h
2nd Gear, Engine revs 2.5, speed 35km/h
2nd Gear, Engine revs 2, speed 29km/h

2- Driving on 4-Low (remember on 4-Hi, your on the same ratio as 2-Hi)

6th Gear, Engine revs 2, speed 22km/h
5th Gear, Engine revs 2, speed 18.5km/h
4th Gear, Engine revs 2, speed 15km/h


Comments:

As said earlier I am going to repeat the test after the upgrade. The speed will always be taken from the GPS to avoid the wrong reading from the odo meter, even if you correct it and supplied the tire size, there will be some difference, some 32” tires are actually 31.7” while others would be 32.2” and so on.

The upgrade here will be 5.13 R&P ratio, and most likely will swap the T-case with the standard Sahara/Sport since its better ratio for desert driving.

Keep an eye on this thread for the results once the second test is performed.


---------------------------------------------------------------------------------------------------------------------------------------
Information from Official Jeep specs document as follows:

TRANSMISSION: NSG 370—MANUAL, SIX-SPEED OVERDRIVE
Availability_________________________________________________________Std.—All models
Description________________________________Synchronized in all forward gears and reverse,
multi-rail shift system with top-mounted shift lever
Clutch__________________________________________________________Hydraulic actuation
Gear Ratios
1st______________________________________________________________________4.46
2nd_____________________________________________________________________2.61
3rd______________________________________________________________________1.72
4th _____________________________________________________________________1.25
5th _____________________________________________________________________1.00
6th_____________________________________________________________________ 0.84
Reverse_________________________________________________________________4.06
Axle Ratio_________________________________________3.21 std., 4.10 opt. (std. on Rubicon)
Overall Top Gear____________________________________2.69 std., 3.44 opt. (std. on Rubicon)

TRANSMISSION: 42RLE—AUTOMATIC, FOUR-SPEED OVERDRIVE
Availability_______________________________________________________________Optional

Description____________________Electronic governor, electronically controlled converter clutch
Gear Ratios
1st______________________________________________________________________2.84
2nd_____________________________________________________________________1.57
3rd_______________________________________________________________________1.0
4th______________________________________________________________________0.69
Reverse_________________________________________________________________2.21
Axle Ratio___________________________________________________________________4.10
Overall Top Gear______________________________________________________________2.83

TRANSFER CASE: NV241 COMMAND-TRAC
Type___________________________________________________________________Part-time
Operating Modes________________________________2WD High; 4WD High; Neutral; 4WD Low
Low Range Ratio____________________________________________________________ 2.72:1
Center Differential Type_______________________________________________________None

TRANSFER CASE: NV241OR ROCK-TRAC
Type_________________________________________________________Part-time, heavy-duty
Operating Modes________________________________2WD High; 4WD High; Neutral; 4WD Low
Low Range Ratio_____________________________________________________________4.0:1
Center Differential Type_______________________________________________________None

TIRES
Availability_________________________________________________________________X Std.
Size and type ___________________________________P225/75R16, on/off-road, black side wall
Mfr. and model________________________________________________Goodyear Wrangler ST
Revs per mile (km)______________________________________________________ 712 (1146)
Availability______________________________________________________________X Optional
Size and type___________________________________P245/75R16, on/off-road, black side wall
Mfr. and mode_______________________________________________Goodyear Wrangler SRA
Revs per mile (km)______________________________________________________687 (1106)
Availability________________________________________________________Sahara Standard
Size and type_____________________________P255/75R17, on/off-road, outlined white lettered
Mfr. and model______________________________________________Goodyear Wrangler SRA
Revs per mile (km)______________________________________________________661 (1064) Availability_________________________________________________________Sahara Optional
Size and type_____________________________P255/70R18, on/off-road, outlined white lettered
Mfr. and model_______________________________________________Bridgestone Dueler 693
Revs per mile (km)______________________________________________________653 (1051)
Availability________________________________________________________Rubicon Standard
Size and type__________________________________LT255/75R17, on/off-road, black side wall
Mfr. and model_____________________________________________B.F. Goodrich Mud Terrain
Revs per mile (km)______________________________________________________650 (1046)

Thank you Chief, looking forward to your next post. What is main reason you went for the 5.12 and will you be upgrading the front axles and universal joints? Please keep us updated as this is a very interesting topic.

I went to 5.13 to match the 35" tires and keep me in the power band. As you know the Rubicon axles are stonger than the Sahara/sport model they are 32 spline heavy duty, so they take the load.

Additionally I have done the reinforcement of the axle tube and the knockles to avoide bending from JUMPS http://project-jk.com/jeep-jk-write-ups ... nt-gussets

Thanks Khaiwi, very good info - you gonna have masive torque 8O

halwa wrote:Thanks Khaiwi, very good info - you gonna have masive torque 8O

It's being built with the name "LIWA-Rubicon" in mind,

Hi Khaiwi

we r still waiting for the feed back !!! any new results?

That Jeep is awsom, you try and follow him in Liwa and you will see for your self hehehehehehe :veryhot:

One note of caution on re-gearing the differential by changing ring and pinion: this is an installation that requires very precise measurement and should be done with due care.

The method to align the teeth properly involves a coloured dye - so that's the first thing to check with your mechanic. Does he plan on using a coloured dye or does he think he can go by "feel".

By feel does not work, regardless of how much experience he has. If the gears do not match perfectly, in 6 months you'll have worn them down.

Here is a guide how it's done, so you can have some idea before handing over to the mechanic:

http://www.planet4x4.net/forums/dana60gear.php

And this is what the dye looks like:

this is the calculator i use, it is simple and complete.
give it a try guys and it will drive you nuts !!!
http://www.novak-adapt.com/knowledge/gearing.htm

PR wrote:One note of caution on re-gearing the differential by changing ring and pinion: this is an installation that requires very precise measurement and should be done with due care.

The method to align the teeth properly involves a coloured dye - so that's the first thing to check with your mechanic. Does he plan on using a coloured dye or does he think he can go by "feel".

By feel does not work, regardless of how much experience he has. If the gears do not match perfectly, in 6 months you'll have worn them down.

Here is a guide how it's done, so you can have some idea before handing over to the mechanic:

http://www.planet4x4.net/forums/dana60gear.php

And this is what the dye looks like:

Absolutely mate. And if you wanna do a perfect job, you have to measure your backlash. Getting the teeth to mesh properly was a pain the first attempts , then you get used to it. I settled for this But as for re-gearing equations, I haven't looked this up yet but maybe someone can give his thoughts: since you need this extra torque at the wheels to turn bigger tires, would the same be required ..say if i'm running 3.73 and an upgraded engine that produces 300 lb of torque???? assuming running 33's. I know some would say your axles might go first thing, but this is manageable with being easy on the pedal.

Great thread!!

great thread Khaiwi

i found this rpm chart for the auto and manual JKs



hope it benefits

Hi there Chief.

I have uupgraded my suspension with the Icon stage one lift kit. This prompted me to get a set of 285/75/16 BFG's.

I know feel the heavier/ bigger tires are robbing the car of performance, especially when climbing dunes. On the road its OK.

I am looking to regear to 4.56 and was wondering if you would care to advice on this. Is regearing a good option to start with. Who in your opinion can doa good job with it in the UAE.

Also i used to air down to 12 PSI fron and 10 PSI rear on my stock 265/75/16 Dunlops and I did the same last weekend on my 285/75/16 BFG's and I noticed a serious power loss. Coud this be partically corrected by say airing down to 20 PSI front and 18 PSI rear as the tires are bigger and the tread is more agressive than the stock dunlops.

Question:

I changed my tires to 33" and did no re-gear.

Performance wise it is ok. Ofcause it would be better after regearing but due to the fuelmilage etc. I am not in hurry with that. Maybe for the next season.

My question now is:
If I have big tires and low gear ratio (stock), does my differential gets damaged due to the force from the enging and the big tires wich grearting a, I would call it "back force".

Forgot to mention.

I have a automatic transmision and drive manly in 4H