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

?

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

  1. 35"

    12.1%
  2. 37"

    55.3%
  3. 40"

    29.8%
  4. None of the above

    2.8%
  1. nimbler

    nimbler Member

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    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!
     
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  2. Alf1096

    Alf1096 Active Member

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    In my opinion it has nothing to do with crawl ratio. It’s all about maintaining highway speed without high rpms or shifting all over the place.
     
  3. OP
    OP
    nimbler

    nimbler Member

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    Valid point. The question is however is not so much on the crawl ratio, as it is in maintaining the factory designated ration. I.e., to adjust the ratio according to the tire size change in order to maintain (not increase) the torque pressure on the mechanical circuit, axels, powetrain, and so on; maintaining the "ratio balance". Would that not also then imply that the transmission will remain happy and use all 8 gears in the same way it used to with the factory tires?
     
  4. Amric

    Amric Active Member

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    You are assuming that the engine torque,8 speed gears, 4.10 ratio, and 33” tires were the only combination that works perfect, and Jeep didn’t just pick from the parts bin, but matched each gear exactly. If they didn’t have an 8 speed they would have used a 5 or 6 speed, and we would need to gear more. The manual has different gears, and the 2.0l has different power and, but any of the 4 combinations work with the same tires and gears. Point I am trying to make is pick the gear that meets your needs, don’t just match the factory gearing. For me, if I was to regear, I would like highway RPM in the 2200-2600 in my highest gear, and would do the math for that.
     
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  5. OP
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    nimbler

    nimbler Member

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    This makes sense. I'm going to use the grimmjeeper.com site and see if I gain any new insights into this.
     
  6. vavaroutsos

    vavaroutsos Well-Known Member

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    If you go to larger tires, re-gearing is not going to reduce stress on the axles. It will help components upstream in the drive train (xfer case, transmission, etc.).
     
  7. OP
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    nimbler

    nimbler Member

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    That is absolutely correct.
     
  8. butcho

    butcho Well-Known Member

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    This ^!

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

    vavaroutsos Well-Known Member

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    At 70 mph, you are looking at about 1734 rpm with the manual and about 1614 rpm with the auto. This is using a dynamic tire diameter of 33.7" for your 35" tire.
     
  10. butcho

    butcho Well-Known Member

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    I don’t get it.
     
  11. vavaroutsos

    vavaroutsos Well-Known Member

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    Don't get what, dynamic tire diameter? The tire doesn't stay round when it rolls. The bottom flattens out reducing the effective rolling diameter. Most tire manufactures will give a rev/mile number for the conditions specified. If you calculate what diameter that corresponds to, you will see it is usually about 1" less than the static diameter for tires with the aspect ratio and size we are talking about on a Jeep. With your 20" wheels it might be a little less, but it's not going to make much difference, your gearing is way too tall in my opinion, especially with a manual transmission.
     
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  12. OP
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    nimbler

    nimbler Member

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    So another factor in deciding when to regearing is the effect of the tire size on the engine revolution rate. Putting a 37" on a JL Rubicon is a ~12% increase in tire size (assuming the 37" is actually 37" for simplicity). That means, that to maintain the same speeds in the same gears, you would be revving 12% lower. For the 8-speed automatic transmission (850RE (~ ZF's 8HP50)), the numbers look like so on the stock 33" tires:

    G 1600 2000 3000
    1 8 10 14
    2 12 14 22
    3 17 22 32
    4 22 27 41
    5 28 35 53
    6 36 45 68
    7 43 54 81
    8 54 68 102


    The first column is the gear, the second row is the MPH @ 1600RPM, the third row is the MPH @ 2000RPM, and so on.

    Now if I switch tires to a 37", these all increase by the same 12%:

    G 1600 2000 3000
    1 9 11 16
    2 13 16 24
    3 19 24 36
    4 24 30 46
    5 32 40 60
    6 41 51 76
    7 48 61 91
    8 61 76 114


    I picked those RPM numbers based on most people stating that on highways they like to be sitting at anywhere from 2000 to 2500 RPM.

    How would you determine if the transmission is okay with the change; would it ever make it to 8th gear, would it get confused? How would you go about figuring out if the transmission is okay with this, or if it would be in need of regearing (or something else)?
     
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  13. DesertsJL

    DesertsJL Well-Known Member

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    Regearing isn’t just about getting the RPM ratio back to stock, it’s about handling the larger tire AND additional rotating mass of the tire. Even with 35” tires my Jeep was struggling with using the upper transmission gears.
     
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  14. OP
    OP
    nimbler

    nimbler Member

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    This is what I'm trying to tease out... You say that that the Jeep was struggling using the overdrive gears... how would you counter that, obviously without reverting the tire size back to stock. Wouldn't regearing to restore original powertrain ratios do most of the work there? Are there downsides other than cost?
     
  15. DesertsJL

    DesertsJL Well-Known Member

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    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.
     
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