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

[DesertsJL]

This ^!

Someone do the math for me. 35” tires, 20” wheels. 3.45 gears

The stock gear ratio of 3.45 was way too low to begin with and it was marginal at best for the stock tires. Going to 35” tires on your JL would require gears for the tire size and weight regardless of the model. Suggest 4.88.

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[butcho]

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’m totally having a hard time understanding all this. I live in the piedmont (rolling not steep hill) area of NC and I do go 70-80 mph on the highway. What should do do then?

 

[DesertsJL]

I’m totally having a hard time understanding all this. I live in the piedmont (rolling not steep hill) area of NC and I do go 70-80 mph on the highway. What should do do then?

I was just explaining why I went so deep on my gears.

What rpm do you want to be turning at 80? It’s easy to figure out a gear ratio from there. For example, at 80mph with 35” tires and 4.88 gears you would be turning 2511rpm. That’s a very comfortable engine speed and it would get you back in the power band and allow you to use all 8 gears of your transmission.

 

[butcho]

I was just explaining why I went so deep on my gears.

What rpm do you want to be turning at 80? It’s easy to figure out a gear ratio from there. For example, at 80mph with 35” tires and 4.88 gears you would be turning 2511rpm. That’s a very comfortable engine speed and it would get you back in the power band and allow you to use all 8 gears of your transmission.

Very helpful. I was considering going from 3.45 to 4.10 but 4.5 or 4.8 sounds like a better choice. My goal is to remove the sluggishness and recover my MPG. 4.88 still recommended?

And where do you get this wonderful gear ratio? I'm having a hard time finding parts.

 

[DesertsJL]

Very helpful. I was considering going from 3.45 to 4.10 but 4.5 or 4.8 sounds like a better choice. My goal is to remove the sluggishness and recover my MPG. 4.88 still recommended?

And where do you get this wonderful gear ratio? I'm having a hard time finding parts.

https://www.jlwranglerforums.com/forum/threads/dana-advantek-axle-brochure.8030/

The part numbers are in the Dana/Spicer brochure. You’ll need to know which axles that you have depending on options and order accordingly. Absolutely recommend 4.88 for your set up.

 

[butcho]

https://www.jlwranglerforums.com/forum/threads/dana-advantek-axle-brochure.8030/

The part numbers are in the Dana/Spicer brochure. You’ll need to know which axles that you have depending on options and order accordingly. Absolutely recommend 4.88 for your set up.

Need all these parts?


Spicer Part # | Sport/Sahara bundle - 4.88 Ratio
10026639 | Front Dana 30™ Ring and Pinion - 4.10 Ratio
10067231 | Rear Dana 35™ AdvanTEK® Ring and Pinion - 4.10 Ratio
10040313 | Master Overhaul Kit Jeep Wrangler JL Dana 30™ Front
10040452 | Master Overhaul Kit Jeep® Wrangler® JL Dana 35™ AdvanTEK® Rear

 

[DesertsJL]

Need all these parts?


Spicer Part # | Sport/Sahara bundle - 4.88 Ratio
10026639 | Front Dana 30™ Ring and Pinion - 4.10 Ratio
10067231 | Rear Dana 35™ AdvanTEK® Ring and Pinion - 4.10 Ratio
10040313 | Master Overhaul Kit Jeep Wrangler JL Dana 30™ Front
10040452 | Master Overhaul Kit Jeep® Wrangler® JL Dana 35™ AdvanTEK® Rear

Yes, but first verify which rear end that you have. You may have had an option for the Dana 44 rear which would be different R&P and overhaul kit. You need the overhaul kits for the shims, crush sleeve, nuts, and other hardware.

 

[nimbler]

To make the calcs a little simpler to visualize, here's a quick interactive graph for the 8 gears of the JL:

https://www.desmos.com/calculator/5fkhxvb3og

w - The tire size in inches
D - the diff ratio: d corresponds to D, that is, [1,2,3,4] correspond to [4.10, 4.56, 4.88, 5.13]
T - the 2 states of the transfer case: 1 is L (1:4), and 2 is H (1:1)

What it does not account for is the rotational mass effects resulting from the larger tires (see @DesertsJL's comment immediately above this post).

Here's a look at the JL stock, and @DesertJL's two suggestions:
1. 4.10 @ 33": https://www.desmos.com/calculator/hfbditv450
2. 4.88 @ 35": https://www.desmos.com/calculator/tunanhwni7
3. 5.38 @ 37": https://www.desmos.com/calculator/kanrlanh26

 

[mickey3]

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!

No need to really over think this. Lower gears are great fun and make for a sweet ride! You will swear the engine has more power and that the Jeep is 500 lb lighter too. Modern Jeeps have crazy overdrives (for EPA purposes) and they do not really work well outside of the EPA cycle. In real life, a Jeep will probably get better fuel mileage after you regear, as it will be in top gear much sooner and will downshift way less. I have regeared our current Jeep JLKUR (5.13s) and several cars in the past, never any regrets.
As soon as they are available I will put 4.10s in the ZO6!

 

[butcho]

I know this thread

I was just explaining why I went so deep on my gears.

What rpm do you want to be turning at 80? It’s easy to figure out a gear ratio from there. For example, at 80mph with 35” tires and 4.88 gears you would be turning 2511rpm. That’s a very comfortable engine speed and it would get you back in the power band and allow you to use all 8 gears of your transmission.

Forgot the transfer case. This thread assumes a 4:1 Rubicon transfer case but I have the sport which is 2.72:1. Does this effect anything?

To make the calcs a little simpler to visualize, here's a quick interactive graph for the 8 gears of the JL:

https://www.desmos.com/calculator/5fkhxvb3og

w - The tire size in inches
D - the diff ratio: d corresponds to D, that is, [1,2,3,4] correspond to [4.10, 4.56, 4.88, 5.13]
T - the 2 states of the transfer case: 1 is L (1:4), and 2 is H (1:1)

What it does not account for is the rotational mass effects resulting from the larger tires (see @DesertsJL's comment immediately above this post).

Here's a look at the JL stock, and @DesertJL's two suggestions:
1. 4.10 @ 33": https://www.desmos.com/calculator/hfbditv450
2. 4.88 @ 35": https://www.desmos.com/calculator/tunanhwni7
3. 5.38 @ 37": https://www.desmos.com/calculator/kanrlanh26

Same question although the T doesn't seem to agree with this.

 

[DesertsJL]

I know this thread


Forgot the transfer case. This thread assumes a 4:1 Rubicon transfer case but I have the sport which is 2.72:1. Does this effect anything?



Same question although the T doesn't seem to agree with this.

Honestly, I don’t care about the off-road ratio as I rarely would ever hit the limits of the speeds available. I’m more worried about everyday driving which is a 1:1 ratio.

 

[butcho]

Honestly, I don’t care about the off-road ratio as I rarely would ever hit the limits of the speeds available. I’m more worried about everyday driving which is a 1:1 ratio.

Ok so the differences in transfer case between the rubicon and non-rubicon doesn’t matter for this equation. Thanks!

 

[nimbler]

Hwy guys, I've updated the graph - I'm still interested in keeping this thread going, so am trying to get a UI for playing around with the settings meanwhile:

https://www.desmos.com/calculator/ydhainzrcc

The speed is in MPH down along the (horizontal) x-axis, and the RPM is along the (vertical) y-axis as before.

Updates from last graph:
- You will see 8 lines depicting each gear. The time at which the transmission changes gear is completely arbitrary in this graph - look for the graph called m(x) and play with it to suit how you think you will be driving, and let me know if I can add more sliders/options to better model this. The current one is *completely arbitrary*.
- The D and T sliders will go through the differentials (4.10,4.56,4.88,5.13,5.38) and transfer case (4.0,1.0) respectively. I added the 2.72 transfer gear ratio to the T slider.

This may be easier to use, at least to visualize, than the tooling in grimmjeeper site.

Next I think the horsepower and torque curves would be useful to have on there, so you can superimpose your driving against the graph.

The initial setting of the graph model a stock JL Rubicon, but again, the m(x) graph (when you would change gears) is pull out of the air.

 

[Fire Burns]

This poll is missing 38 inch tires, which would have been my selection

 

[JB3]

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?

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