Gears for 37" tires

  1. Kredit

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    What is the best ratio for gears with 37" tires? I have 35" on my JKU with 4:56 gears and it's a great combo.So i'm thinking I would use 4:88 for the 37" tires on my Gladiator.
     
  2. BrandRezake

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    I am really glad you started a thread based on this topic. I have been searching for comments on the forum about this.
    I had 4:56 on my JKU (Manual Transmission) with 35s, and while they were amazing, I wished they were 4:10 sometimes when traveling.
    I am very curious what the 37s feel like on the JT with the Automatic transmission. Anyone with the JT and 37s want to chime in?
     
  3. Hemi

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    I have 4.88 gears on my JK Rubi auto w/Pentastar and could go up a tire size no problem. If that helps
     
  4. Renegade

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    I have 4.88 in my JK with 37's, but from what I've read on the JL Forum, most people feel like the new 8-speed transmission makes gear swaps less of a necessity. I plan to try out 37's on the factory 4.10s before considering anything.
     
  5. Ole Cowboy

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    4:88, got almost 100K on 37's and 4:88's...even with that there have been times I had to run in 5th and even 4th a few times when the headwinds get bad like I 10 just W of the Ca border heading into LA.
    If you are asking about SERIOUS HD wheeling in Moab on the 4+ trails and you hit them often like I did I would not hesitate to go 5:13 with 37's.

    I looked into running 39's or 40's but when you steep up that big you open up a whole 'nother can of worms ...I strongly believed that you drove your rig to where you wheels. I will tell that most guys that haul their rigs do so 'cause it gonna break down on them.

    If you want to just peddle your rig for the Sonic crowd you can get by on 4:10 but the PAIN will be on the hwy, if you don't need to run 70 mph, stay with your 4:10 and move to 4:56. Even with my 4:88's I had a lot of hwy pain and so I added a SC which made all the diff in the world at 70 mph driving out to Ca, AZ, Utah etc.

    Here is what I ran, disregard the CAI setup, I was still experimenting and I later changed the design and pulled air thru the back side of the passenger headlight bucket...

    P9040007.jpeg
     
  6. Renegade

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    Was that in a manual transmission TJ? Sounds like a great TJ setup, but things with the 3.6 and auto are a lot different.
     
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  7. Slapping_Rabbits

    Slapping_Rabbits Well-Known Member

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    What about gearing 7.17s? How would that drive?
     
  8. gsxr310

    gsxr310 Active Member

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    For 37's, definitely 4.88.
     
  9. Ole Cowboy

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    There are constants: At each corner, you have a rotating mass of around 125-135 lbs and be it CJ, TJ, LJ, JK, JL or JT, it makes no difference that has to be overcome and the faster you go the more of a factor it becomes.

    The more gears you have access to spread out across the RPM band the better to deal with that rotating mass, but the higher up you go in gear the more that rotating mass affects you as wind and road resistance is exponential IIRC. The 3.6 comes alive in the higher end of its rpm band and that may be a good thing.

    On the 3.6: WHY Jeep used that engine and continues I have no idea, it is NOT a fit for a Jeep where rocks and off-road trails require way down low torque. Jeep picked it because it gets great mpg, I had no problem getting 19 mpg out of my Rubicon JK. The best ever engine for the Jeep was the old straight 6 in spite of its boatload of disadvantages, but if its trail performance you want it fits the bill.

    I keep saying I am going to repower one of my Jeep with the GM 5.3 Liter V8 EcoTec3 L83 Engine


    Reg gas, 380 HP/416 T @ 4100 rpm, got the low-end grunt, will give you a solid 22 mpg on highway, surely Jeep or Chry has SOMETHING in the line up that works better than the 3.6...
     
  10. Ole Cowboy

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  11. LidLess07X

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    When comaparing to JK ‘s, it is very important what year and engine/trans you have in your JK

    Huge difference between the 3.8 and the 3.6
     
  12. OP
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    Kredit

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    My JKU is a 2013 automatic with a 3.6. I had a 2006 with the straight 6 that was a great motor. I used the chart back when I did my 2013 just didn’t know if the 8 speed tranny would make 4.56 a better all around choice for 37s
     
  13. planman

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    #13 Jun 6, 2019
    Last edited: Jun 6, 2019
    With the 850RE automatic, stock Rubi 4.10 gears, and stock 285/70R17 tires that measure 32.8" unmounted and likely around 32" mounted and under full load, these are the specs in the attached pics.

    ( I ran a gear ratio calculator on grimmjeeper .com)

    With 37x12.50R17s that run 36.7 unmounted, they might end up at 35.5" mounted under full load.

    Using the calculator, 4.56 gears would result in almost the same engine RPMs in 8th gear at highway speeds.

    However, running a heavier, taller tire, you generally want the extra horsepower of having your engine run at higher rpms at highway speeds.

    4.88s would put you at 2166 rpm at 70 mph, and 2476 at 80 mph.

    My sense is that if you are going to re-gear, you'd want to at least do 4.88s because the extra maybe 147 rpm at 70 and 169 at 80 will help produce slightly more horsepower without killing fuel economy.

    5.13s would put you at 2277 at 70 mph, and 2603 at 80 mph.

    That results in 258 rpm over stock at 70 mph, and 296 rpm over stock at 80 mph.

    My sense is that if you have heavy accessories, if you tow, or if you live at a higher elevation above sea level or in a mountainous area, you'd likely be happier with the 5.13s because it is probably worth it to you run 258-296 higher engine rpm on the highway to produce enough horsepower to reduce the frequency of the transmission downshifting.

    What kills fuel economy is when an automatic transmission continuously "hunts" between gears, or frequently downshifts because the top gear doesn't produce enough horsepower to hold that gear in the wind, hills, etc.

    Screen Shot 2019-06-06 at 5.29.45 PM.png

    Screen Shot 2019-06-06 at 5.30.06 PM.png

    Screen Shot 2019-06-06 at 5.30.45 PM.png
     
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  14. OP
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    Kredit

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    Thanks that’s great info I believe I’ll go with 4.88 and 37” most 4w is below 5000 ft in Arizona
     
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