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Regearing Math - What am I missing?

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Old 06-04-2017, 07:01 AM
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Default Regearing Math - What am I missing?

I have a 2015 JKU Rubicon that came from the factory with 3.73 gears. From what I've read the way to calculate the needed gear ratio when upgrading tires is as follows.

New Tire Height x Current Gear Ratio / Previous Tire Height.

If that's correct then my math is as follows.

34.4 x 3.73 / 32.1 = 3.99

The closest I can find to a 3.99 would be 4.10's. Yet everyone I read about either winds up with 4.56's or 4.88's. Am I doing my math wrong? Or is there some intangible I'm not quite grasping?
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Old 06-04-2017, 07:20 AM
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I have never heard of this calculation before. Not saying it does not work, just never heard of it before. Search for "2012+" gearing chart and you will see a graph that will show you the ideal gearing for your tire size. 4:10 will work but 4:56 will be much better.
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Old 06-04-2017, 07:43 AM
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http://www.4lo.com/calc/gearratio.php

This has the correct calculations.
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Old 06-04-2017, 07:59 AM
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Originally Posted by 4x4chick View Post
I have never heard of this calculation before. Not saying it does not work, just never heard of it before. Search for "2012+" gearing chart and you will see a graph that will show you the ideal gearing for your tire size. 4:10 will work but 4:56 will be much better.
I've seen the chart. My original plan was to go with 4.56's. If I had started with 4.10's I think the math would work great on that.

34.4 x 4.10 / 32.1 = 4.39

At 4.39 the closest ratio the Dana 44 accepts would be 4.56 unless I'm mistaken.
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Old 06-04-2017, 09:56 AM
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Too much math for me....LOL. Fortunately there are only a few gearsets to choose from, and it's all going to depend on the tires your running. Most with 35s and the 3.6L end up at 4.56........unless you think there is a chance that you're going to go to a 37 or wheel really aggressively. If you currently have 3.73s, going to 4.10s isn't going to be a fart in the wind difference. You really need to jump 2 spots to make the cost outlay worthwhile, which would mean going to 4.56. It sounds like 4.88s are going to be too aggressive for you, making the decision a whole lot easier.

Just my opinion.
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Old 06-04-2017, 10:06 AM
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Your formula keeps RPM the same for road speed, (step 1). Why people would choose a 4.56 instead is to overcome the increased rotational mass of the larger tires (need more torque to get them spinning).
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Old 06-04-2017, 10:16 AM
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Originally Posted by zstairlessone View Post
Your formula keeps RPM the same for road speed, (step 1). Why people would choose a 4.56 instead is to overcome the increased rotational mass of the larger tires (need more torque to get them spinning).
And THERE's the bit I was missing. I figured it was some such, but assumptions have come back to bite me before. So people go for the shorter gearing to overcome the increased rotational mass of the heavier tires by moving the engine back into the better part of the power band. Makes complete sense.

4.56's it is. I'm not a heavy wheeler nor do I want 37's so I can just ignore the 4.88's and go with the higher MPG (on 35's) of the 4.56's. Thanks gang!
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Old 06-04-2017, 10:16 AM
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4.10 use to be the standard gear size for the Rubicon, jeep dropped it to 3.73 and reduced off road performance to gain a mpg or two. There are different levels of performance and what you are going to get out of a gear set. If you are climbing boulders and steeps then you want lower gearing, if you are driving fire roads and dirt trails or running faster thru the desert then you may want a higher gear.
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Old 06-04-2017, 10:14 PM
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35 x12.50. i went with 4.56 because i wanted the stockish Rubi feel back and i dont do any real crawling.
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Old 06-17-2017, 01:38 PM
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Originally Posted by Skippman View Post
I have a 2015 JKU Rubicon that came from the factory with 3.73 gears. From what I've read the way to calculate the needed gear ratio when upgrading tires is as follows.

New Tire Height x Current Gear Ratio / Previous Tire Height.

If that's correct then my math is as follows.

34.4 x 3.73 / 32.1 = 3.99

The closest I can find to a 3.99 would be 4.10's. Yet everyone I read about either winds up with 4.56's or 4.88's. Am I doing my math wrong? Or is there some intangible I'm not quite grasping?
I'd rather use this:

34.4/32.1=7.2% increase
3.73*1.072=3.99

So realistically, your calculation is totally correct, just harder to understand why.

I would go with 4.10s or leave it alone
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