Sponsored

JLU Rubicon Auto: 35" and 37" - Regearing Math

Starting at what tire size (on the JLUR) would you regear?


  • Total voters
    288

vavaroutsos

Well-Known Member
First Name
Pete
Joined
Nov 24, 2017
Threads
9
Messages
621
Reaction score
413
Location
Scotts Valley, CA
Vehicle(s)
2015 Sprinter Crew Van 2500 High Roof 144" WB OM651 Brilliant Silver, 2019 JLR 3.6L 6SPD Granite Crystal
How does this change for a manual rubicon.

I’m running 35s on the stock 4.10 right now. I’m upgrading to 37s soon and plan to regear next week to 4.88.

Thoughts?
Go 5.13. With the stock 4.10 gearing and 33" tires I find I don't use 6th gear on the manual transmission unless I'm on a downgrade. If you want 6th gear to feel close to the stock 5th gear, then 5.13 gearing with 37" tires will be close. This will give you a really short 1st gear for crawling.

It seems the automatic transmission handles non-ideal gearing much better than the manual. With 8 gears the ratios are tighter, and the torque converter helps at low RPM. On the manual transmission both 5th and 6th are overdrive gears. I think it would have been better with only 6th being an overdrive gear and having a slightly tighter ratio between the gears, but there's not much you can do about that.
Sponsored

 
Last edited:

Peanut Butter

Well-Known Member
First Name
Pat
Joined
Mar 4, 2018
Threads
4
Messages
170
Reaction score
264
Location
Boise, ID
Vehicle(s)
JL?
How does this change for a manual rubicon.

I’m running 35s on the stock 4.10 right now. I’m upgrading to 37s soon and plan to regear next week to 4.88.

Thoughts?
Did you ever regear?

If so, what did you decide?
 

jimlove9771

Well-Known Member
First Name
Jim
Joined
Dec 11, 2018
Threads
2
Messages
95
Reaction score
80
Location
Spring, Texas
Vehicle(s)
2018 Jeep Wrangler JL Sport S
Occupation
Retird
Let’s say that I only wanted to keep the original relative gear ratio between the stock tire and gear and the larger new tire and gear. That would mean that roughly a 4.35 gear would be needed for 35’s and 4.60 gears would be needed for 37” tires. The problem is that with larger tires you also have much more rotating mass, necessitating a deeper gear than the calculator would reflect. You need more gear in order to control and move the extra weight of the tires.

My suggestion is to go at least one step deeper than the calculator would suggest. For 35” tires use 4.88 and for 37” tires use 5.13 gears.

For me, I went very deep with 5.38 gears and currently 35” tires, admittedly a little too deep. However I live in a mountainous area and rarely drive over 70mph. Due to the super tall 8th gear of a .67 overdrive, at 70mph I’m running 2450rpm. My plans are to run a larger tire down the road.
I have totally different goals in mind than your average owner. My goal is to create a sleeper street racer! I know this sounds ludicrous but it’s my choice. I have a 2018 JL (2 door). I upgraded to 315/70 R17 BFG K02’s with a Mooar 2” lift. Here’s where I depart from almost all of you. I have just completed installation of Prodigy Performance Stage 2 turbo kit. With th3 stock 3.45 ratio I am not happy wirh the 0-60 acceleration. I don’t care about gas mileage but I don’t want to damage the drive train. I have the 8 speed auto on a Sport S. I am considering the 4,88 ratio. Any suggestions.
 

vavaroutsos

Well-Known Member
First Name
Pete
Joined
Nov 24, 2017
Threads
9
Messages
621
Reaction score
413
Location
Scotts Valley, CA
Vehicle(s)
2015 Sprinter Crew Van 2500 High Roof 144" WB OM651 Brilliant Silver, 2019 JLR 3.6L 6SPD Granite Crystal
I have totally different goals in mind than your average owner. My goal is to create a sleeper street racer! I know this sounds ludicrous but it’s my choice. I have a 2018 JL (2 door). I upgraded to 315/70 R17 BFG K02’s with a Mooar 2” lift. Here’s where I depart from almost all of you. I have just completed installation of Prodigy Performance Stage 2 turbo kit. With th3 stock 3.45 ratio I am not happy wirh the 0-60 acceleration. I don’t care about gas mileage but I don’t want to damage the drive train. I have the 8 speed auto on a Sport S. I am considering the 4,88 ratio. Any suggestions.
If you only care about 0-60 acceleration, I would go even shorter, maybe 5.13.
 

Sponsored

XJrider

Banned
Banned
Banned
Joined
Oct 21, 2018
Threads
1
Messages
492
Reaction score
288
Location
North East
Vehicle(s)
Rubicon JL , 2 JK’s , XJ
The problem statement is: determine what is the best course of action (w.r.t regearing axels) if one desires to move to either 35" or 37" tires.

I apologize for the length of the question, it is to avoid any back and forth on the math, so we can stick to the question. I'll change the color of the boring math so that you can skip it, and only refer to it as required; feel free to skip over it

---

From my (very limited) understanding, the math is relatively simple when it comes to the gear setup:

For the Jeep JLs we have three components in this mechanical circuit:

  1. a transmission (850RE AUTOMATIC, EIGHT-SPEED OVERDRIVE),
  2. a transfer case (NV241OR ROCK-TRAC),
  3. and an axel differential (Dana 44).
In the Rubicon, these present the following ratios respectively (taken from https://www.jlwranglerforums.com/fo...-jeep-wrangler-jl-specs-info-wallpapers.2073/):
  1. 4.71 in first gear
  2. 4.0 in low range
  3. 4.10
That means that the crawl ration (in first gear, in low) is:

4.71 x 4.0 x 4.10 = 77.244

Now, the JL Rubicon was designed for that ratio on 33" tires. A 35" is 6.061% larger in diameter, and the 37" is 12.12% larger when compared to the stock/standard 33". Since torque is linearly proportional to the radius/diameter, that means that torque that the circuit needs to drive is also 6.06% and 12.12% more, respectively, for these tires.

In order to relieve the powertrain of this additional torment, one would simply adjust the gear ratio accordingly in order to maintain the original 77.244 ratio. And that would take place in the Dana 44 gears.

One thing to correct for before moving forward: the Dana 44's 4.10 gear ratio is calculated from the ratio between its 45 ring gear teeth, and its 11 pinion teeth, so it's more accurate figure is 45 / 11 = 4.091. Therefore the actual crawl ratio with that figure is:

4.71 x 4.0 x 45 / 11 = 77.07

Also note that for this calculation, the other two numbers (transfer case and transmission) do not matter as they remain constant here.

Now we can solve for the 35" and the 37" gear ratios respectively:

35" tires: (35 / 33) x (45 / 11) = 1575 / 363 = 4.339
37" tires: (37 / 33) x (45 / 11) = 1665 / 363 = 4.587

From my understanding, it's not important at all to get an exact match (many are in fact running 37"s without any issues), but if you do change gear ratios, it's better to go higher than calculated, rather than lower.

Assuming that we will remain with Dana for now (see http://www.sierragear.com/ring-and-pinion-gear-sets-dana/), then the best new ratios would be the Dana 44 4.55, and the Dana 44 4.89 respectively. Our new adjusted (account for tire change) crawl ratios (and percentage change from stock design) would be:

35" tires: 4.71 x 4.0 x 4.55 x 33 / 35 = 80.8 (4.38% increase in effective crawl ratio)
37" tires: 4.71 x 4.0 x 4.55 x 33 / 37 = 82.2 (6.11% increase in effective crawl ratio)

Note that the effective crawl ratio takes in to account the change in the tire, actual new crawl ratios would be obviously higher due to the regearing: 85.72 and 92.13 respectively.


---

With all that crap aside, would you regear to maintain factory ratio? Yes, the Rubicon can run the 35" and 37" without any apparent issues, but surely over the lifetime of the vehicle, it would have a noticeable impact, no? There's considerably more torque request on the axels, the powertrain components, the gears all the time.

I'm a neub, no experience in off-roading, in jeeps, nothing. And, I'm about to spend a lot of money on a JLUR Automatic. I like the 35"s, I love the 37"s + the MOPAR lift. I want to keep this vehicle for a very long time, so longevity is important. I also do plan on doing some off-roading, so this won't be a vanity/Nordstrom build.

Looking forward to your thoughts!
Great job on the math! If you are goong to run 37’s and you drive a lot of mountain pass or unflat runs or a lot of highway, you really cant go wrong with bumping up to the 4.89. Dont forget its a permanent change so if you want to slap those 33’s back on its going to spool up a lot.
 

JC7

Well-Known Member
First Name
Jdog
Joined
Nov 21, 2018
Threads
21
Messages
682
Reaction score
838
Location
Outer space
Vehicle(s)
Jeep Rubicon
I have totally different goals in mind than your average owner. My goal is to create a sleeper street racer! I know this sounds ludicrous but it’s my choice. I have a 2018 JL (2 door). I upgraded to 315/70 R17 BFG K02’s with a Mooar 2” lift. Here’s where I depart from almost all of you. I have just completed installation of Prodigy Performance Stage 2 turbo kit. With th3 stock 3.45 ratio I am not happy wirh the 0-60 acceleration. I don’t care about gas mileage but I don’t want to damage the drive train. I have the 8 speed auto on a Sport S. I am considering the 4,88 ratio. Any suggestions.
You should def be talking to Prodigy about this too as it will change your tune. I like your goals though and literally can't wait to see how the turbo does.
 

vtech243

Well-Known Member
First Name
Ian
Joined
Mar 25, 2018
Threads
6
Messages
58
Reaction score
104
Location
Boston
Vehicle(s)
Wrangler JL & BMW X3 M40i
RPM.jpg


Hey guys -- took a stab at visualizing this in excel. Basically starting with engine RPM and backing into MPH using gear ratios and math (a little pie in here). I also assume that you lose ~13% of the power throughout the system which was the plug for me to get to my real-world RPM / MPH based on my drive to work this morning. Really struggling with if I should re-gear or not ... my issue is less with the RPM and more with when my auto tranny "chooses" to shift (knew I should have got a 6MT). For the record I already changed my tire size using Tazer JL & running KO2 37s which run a little smaller.

Based on the table above, if I re-geared to 4.56/4.88/5.13, at 70 MPH, I would be at 2,226/2,382/2,504 RPM. Seems like this is in-line with what I have read on other parts of this forum.

Question -- for those that have re-geared -- did you notice a difference in shift points along with the increase in RPMs at higher gears? I don't really care about gas because this is going to be my 2nd car -- hope this chart helps others out.

:jk::jk::jk:
 

vavaroutsos

Well-Known Member
First Name
Pete
Joined
Nov 24, 2017
Threads
9
Messages
621
Reaction score
413
Location
Scotts Valley, CA
Vehicle(s)
2015 Sprinter Crew Van 2500 High Roof 144" WB OM651 Brilliant Silver, 2019 JLR 3.6L 6SPD Granite Crystal
RPM.jpg


Hey guys -- took a stab at visualizing this in excel. Basically starting with engine RPM and backing into MPH using gear ratios and math (a little pie in here). I also assume that you lose ~13% of the power throughout the system which was the plug for me to get to my real-world RPM / MPH based on my drive to work this morning. Really struggling with if I should re-gear or not ... my issue is less with the RPM and more with when my auto tranny "chooses" to shift (knew I should have got a 6MT). For the record I already changed my tire size using Tazer JL & running KO2 37s which run a little smaller.

Based on the table above, if I re-geared to 4.56/4.88/5.13, at 70 MPH, I would be at 2,226/2,382/2,504 RPM. Seems like this is in-line with what I have read on other parts of this forum.

Question -- for those that have re-geared -- did you notice a difference in shift points along with the increase in RPMs at higher gears? I don't really care about gas because this is going to be my 2nd car -- hope this chart helps others out.

:jk::jk::jk:
I'm pretty sure the auto in the JL uses a lockup torque converter, so there probably isn't a 13% loss in RPM at cruising speeds.
 

Sponsored

Rubicon_20

Well-Known Member
First Name
Matt
Joined
Feb 13, 2020
Threads
0
Messages
83
Reaction score
58
Location
Orlando, FL
Vehicle(s)
2020 Jeep Wrangler Unlimited Rubicon
Occupation
EHS
FWIW: here is an online gear ratio calculator I built. Might be helpful to someone. https://docs.google.com/spreadsheets/d/1z1xBG_ATYFeRk88ZZBbFNqvIz71IEqOVl_ynTd7pplY/edit?usp=sharing
If I’m looking at your chart correctly, if I decided to put 37” tires on 20” rims on my JLUR auto, you’d recommend 4.56 gears? I do a mix of city/highway (avg 75mph Hwy) and weekend off-roading.
This was a lot to read on this thread and everyone has a different setup. Suggestions greatly appreciated
 

D60

Well-Known Member
Joined
Apr 1, 2019
Threads
39
Messages
1,615
Reaction score
1,827
Location
CO
Vehicle(s)
JL
Why is everybody mentioning wheel size?

I guess theoretically it could affect static loaded radius but not THAT much.

All that really matters is (tire radius at street pressure) x 2 = effective diameter
 

AnnDee4444

Well-Known Member
Joined
Jan 14, 2019
Threads
49
Messages
4,685
Reaction score
6,273
Location
Vehicle(s)
'18 JLR 2.0
Why is everybody mentioning wheel size?

I guess theoretically it could affect static loaded radius but not THAT much.

All that really matters is (tire radius at street pressure) x 2 = effective diameter
FWIW: A 22" wheel is going to have a lot more impact on rotational inertia on smaller tires. Cruising at highway speed may be fine, but the change in acceleration could influence someone who it trying to decide between two different ratios.

If I’m looking at your chart correctly, if I decided to put 37” tires on 20” rims on my JLUR auto, you’d recommend 4.56 gears? I do a mix of city/highway (avg 75mph Hwy) and weekend off-roading.
This was a lot to read on this thread and everyone has a different setup. Suggestions greatly appreciated
Sorry, I don't have any first hand experience with that setup.
 

RockinMarine

Well-Known Member
First Name
Cary
Joined
Mar 31, 2018
Threads
10
Messages
137
Reaction score
111
Location
Menifee CA
Vehicle(s)
2018 Jeep Wrangler Unlimited Sahara JL, Firecracker Red
Occupation
Retired Marine/ Techician for Disney
Ok, I am going to ask for some clarification on something.
I have a 18 JLU Sahara with a Dana 30 front and 44 rear. It came with 3.45 gears and 32" tires. I added a Mopar 2" lift, and went to 35x12s. I lost 8th gear, and do alot of commuting.(75k miles in 2 years) I drive the hills often with no issues.
I want to research, and am looking at 4.56. But most of the posts are saying 4.88s.
Can you give me the Pro/Cons for both ?
Sponsored

 
 



Top