Calculating re-gearing requirements for 37" - JeepForum.com
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post #1 of 7 Old 05-04-2021, 06:17 AM Thread Starter
mvoltin
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Calculating re-gearing requirements for 37"

I just put 37" (actual 35.5") on 2005 6MT 4.0 TJ and have been learning about re-gearing requirements and little confused about the recommendations online. First, to get few details out of way:

1) this car is never going to leave paved city road, never off-roading, almost never highways, etc. It's for my kid to go to school 3 miles away and back on city roads
2) primary (the only) reason for re-gearing is to make car feel "stock" and easier to start from the first gear.
3) I know I need bigger brakes and exploring those options.
4) There are tons of info and opinions about axle choices and types and hope to keep this discussion only about the re-gearing ratio.


There are lots of discussions with strong recommendations that even 4.56 is not enough for 35" tires and to go with to go 4.88 or even 5.13 but not sure how these numbers are calculated. In order to achieve the same "ground speed", upgrading 28" (stock factory size for 2005 TJ x) with 3.07 ratio to 35.5" would require gearing of 3.96 (226*3.07 / 175). This would give about 27% higher gearing ratio and seems accurate based on my speedometer off-set (actual speed is also about 27% higher than what speedometer shows).

So, if my only objective is to get factory feel (factory ration from engine to wheels), is there a reason to go higher than 4.11?

And, if the 4.11 is sufficient, what is the cheapest route? I was reading I could just reuse Cherokee axle? is it a direct fit? Thank you.

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post #2 of 7 Old 05-04-2021, 07:07 AM
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37s would need 5.13 to even get close to stock 4.1 on 30s. even 5.13 wont pull like it did before. i would think brakes will be the larger issue if it ever has to do a panic stop.

tires never match the selling height in most cases. so the idea that they are not really 37 and more like a big 35, well the 35s would still be 2" less so given what you say that same tire in 35 would be more a big 33. so your gearing will still be in the " as much as a d30 can hold and that is 5.13"

why 37s on a tj that is stock wheel base for street? 37s on short wheel base , new driver. Hope they understand the nature of the beast.... Gearing is always based on street.the fallacy people want to believe is that they are for off road so they wont need it. You have a low range you can use off road, notice how when it doubles down in low you have better power? not enough gear tends to put the strain on the trans and engine since they now have to do the grunt work.

If it is an auto 4 speed you really REALLY need those gears. If its a 3 speed auto no overdrive you wont be able to run those 5.13 at speed with out over revving it. Your gears are a math problem if a 4 speed auto that OD is .76 that is a huge over drive and is why that trans need so much gear. But once done it will get better MPG as well. But if it had a 3 speed it will be far worse since the final gear is only 1:1.

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post #3 of 7 Old 05-04-2021, 09:02 AM Thread Starter
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Thank you. Just to answer couple things first:
1) Yes, the 37" makes no sense, its only for looks. Car is driven at 35mph street for 3 miles each way. Yes, it's less safe but I am hoping that having to drive manual will counterbalance some of the safety issues (at least, in my mind, driving manual is safer since they have to pay attention to the road).
2) the car is quite drivable now but wanted to consider regearing to make starting from the first gear easier for the kid (new to manual transmission).

So, my primary question is still around the calculations - I just want to understand the rationale:

The stock tire is 28" that is 175" circumference. The new wheel is 35.5" that is 223" circumference. Thus, car goes 27% longer with every wheel rotation. Then, gearing should change by 27% to get the same "ground speed" for a given RPM/gear. Thus, original 3.07 would need to be 3.96 in order to get the original gearing equivalent.

Is my math incorrect or the original configuration is severely "undergeared" and should be ignored?
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post #4 of 7 Old 05-04-2021, 11:41 AM
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#1. The jeep was undergeared from the factory, the preferred Tjs came with 3.73s. The 3.07s were to help with mileage.



When I had my TJ I ran 4.56 with 33" tires, measured out at 32.4.


Maybe consider how the stock Rubicon from your year was set up, 30.5 inch tires (245/75R16) and 4.10 gears if you do the math on that you will come up with 4.77. Because your tires are much heavier I would round up to 4.88. That would still conidered undergeared for a TJ on 37s but it is the bare minimum you should be looking at.
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post #5 of 7 Old 05-04-2021, 12:17 PM Thread Starter
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Thanks. It's starting to make sense - I was trying to benchmark everything against 3.07 and that was obviously not a good reference.
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post #6 of 7 Old 05-06-2021, 08:40 AM
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Yep, mine had 3.07s with the 33s when I bought it, the 3 months before I regeared it were terrible.

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post #7 of 7 Old 05-09-2021, 01:11 PM
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4.88s/5.13 most common gear for 35s on a tj. well for axles , we all assume its not a d35 rear on 37s.

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