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RETIRED MODERATER
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Discussion Starter #1 (Edited)
One of the ongoing questions that gets asked is, what differential ratio do I need, for a certain size tire:

3.6 engine:
1. 33' tires, the 4.10 Ratio.
2. 35" tires the 4.56 Ratio
3. 37" tires the 4.88 Ratio

3.8 engine:
1. 33" tires with Manual 4.56 Ratio.
2. 33" tires with Auto 4.88 Ratio.
2. 35" tires with Manual 4.88 Ratio.
3. 35" tires with Auto 5.13 Ratio.
4. 37" tires with Manual 5.13 Ratio.
5. 37" tires with Auto 5.38 Ratio.

^^^The above is a general rule of thumb, guideline!!!!!
The terrain, head winds, elevation, and primary type of your driving, (towing maybe), may justify going to a different ratio, EXAMPLE: 3.6 engine with 37" tires. Primarily off road and Rock Crawling, then the 5.13 Ratio, may be more applicable.

Another example: You are running the 2 door, with out any extra heavy weight (bumpers and winch), level ground and no towing. Then with the 3.8 engine and 33" tires, the 4.10 Ratio maybe more applicable!

Another example: You own the heavier by approx. 400 lbs 4 door. You also have added heavier steel Bumpers with a Winch. Plus you are lifted and are running some of the heavier wheel/tire combo's, then this may justify a different ratio, to help off-set the additional weight.

So what I am saying is, that the above guideline will not apply to all, but will apply to most...:thumb:
 

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One of the ongoing questions that gets asked is, what differential ratio do I need, for a certain size tire:

3.6 engine:
1. 33' tires, the 4.10 Ratio.
2. 35" tires the 4.56 Ratio
3. 37" tires the 4.88 Ratio

3.8 engine:
1. 33" tires with Manual 4.56 Ratio.
2. 33" tires with Auto 4.88 Ratio.
2. 35" tires with Manual 4.88 Ratio.
3. 35" tires with Auto 5.13 Ratio.
4. 37" tires with Manual 5.13 Ratio.
5. 37" tires with Auto 5.38 Ratio.

^^^The above is a general rule of thumb, guideline!!!!!
The terrain, head winds, elevation, and primary type of your driving, (towing maybe), may justify going to a different ratio, EXAMPLE: 3.6 engine with 37" tires. Primarily off road and Rock Crawling, then the 5.13 Ratio, may be more applicable.

Another example: You are running the 2 door, with out any extra heavy weight (bumpers and winch), level ground and no towing. Then with the 3.8 engine and 33" tires, the 4.10 Ratio maybe more applicable!

Another example: You own the heavier by approx. 400 lbs 4 door. You also have added heavier steel Bumpers with a Winch. Plus you are lifted and are running some of the heavier wheel/tire combo's, then this may justify a different ratio, to help off-set the additional weight.

So what I am saying is, that the above guideline will not apply to all, but will apply to most...:thumb:
I vote "Sticky" on this. Nice job.
 

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Sticky says......3.6 motor and 33" tires = 4.10 gears most of the time. Should I avoid the 33s with the 3.6 and 3.73 gears? Tires are 285/70/17s which are 11.5 wide and 32.7 tall.
 

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Sticky says......3.6 motor and 33" tires = 4.10 gears most of the time. Should I avoid the 33s with the 3.6 and 3.73 gears? Tires are 285/70/17s which are 11.5 wide and 32.7 tall.
I am sure others with more experience can answer this better but I think you might be alright. If it were me I would try it and see how it works for you.

I have a 2 door Rubicon with steel bumpers, winch, tire carrier and hard top. It has the 3.6 motor, 35" Nitto Trail Grapplers and 4.10 gears. I also used a Superchips Flashpac to program for gear ratio and tire size (I used the 87 octane tune as well). I had to readjust for tire size by using a GPS to check speed. When I entered the measured tire size the speed odmeter was off. I have been very happy with this combination.
 

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UConn Husky
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Sticky says......3.6 motor and 33" tires = 4.10 gears most of the time. Should I avoid the 33s with the 3.6 and 3.73 gears? Tires are 285/70/17s which are 11.5 wide and 32.7 tall.
I have a 2 door JK 3.6 with 35s on 3.73 auto. It's fine - better than my old JKUR 3.8 with 35s on 5.13s.

It's subjective, try it and see, but think you will be fine
 

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I saw a calculator for this previously somewhere that suggested 3.73 gears can take you all the way up to 36"- no motor was taken into consideration though. If I find that again I would compare and post here too if that's okay.
 

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There is a whole lot more to gearing than "subjective personal preference". Everytime I hear that makes me cringe. Gearing is about mechanics and engineering, not preference. Will your Jeep move with incorrect gearing? Sure. But it's about far more than just moving. Folks should understand that.
 

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Not really.On a Jk with the 3.6 the difference between 3:73 and 4:10 is so close most wouldn't be able to tell. Some prefer lower gears because of where they live or what kind of wheeling they do.Ive ran 3:21s with stock tires and 33s.Then 4:56 with 35s and now 37s. I've been happy with all of them.None of them would had caused any issues for the drive line.They all performed very well. There are people who run 35s on 3:21 gears and are happy with it.No it's not ideal but it's not going to hurt anything unless they go lugging up a mountain pulling a trailer. A 3.6 with 3:21 and 35s will do better than a 3.8 with 4:56 and 33s.There are a lot of variables and one of the big ones is personal preference.

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I'm currently on stock 4.10 gears on 35" BFG Mud's which to me measured out at 33.5"...maybe small 35's??? I like the gearing cruising up,the highway at 70mph at around 2350rpm. I wish I had more aceleration and on a trip from Florida up to east TN I felt I needed more gear.

I'm going to go to 37"...I will go a re-gear too. So what do you all think 4.56 or 4.88? I want more pep but don't want to kill it at 75 mph on the highway.

Suggestions?
 

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I'm currently on stock 4.10 gears on 35" BFG Mud's which to me measured out at 33.5"...maybe small 35's??? I like the gearing cruising up,the highway at 70mph at around 2350rpm. I wish I had more aceleration and on a trip from Florida up to east TN I felt I needed more gear.

I'm going to go to 37"...I will go a re-gear too. So what do you all think 4.56 or 4.88? I want more pep but don't want to kill it at 75 mph on the highway.

Suggestions?
I would recommend 4.56s for your current tires. But if you know you're going to go to 37s then get the 4.88s so you don't have to regear twice. It will be a bit buzzy on the hwy though. Drive on the hwy out of over drive. That will be what it's like to be in od with 4.88s and your current tires.


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One also has to take into account that when the tires are mounted on the Jeep under load they are often an inch or so shorter. So my 33's measure exactly 32" under load and the gear chart for my 6 speed manual with 3.73 gears and the 3.6 engine is right in the green, which is where one wants to be for a daily driver and moderate wheeler. The blue section is the performance gear set and is more suitable for a dedicated off roader as one's rpms would run much higher on the highway then one may want and the yellow section, well that gear set is not great for dd or off road but if one doesn't mind the performance loss then they are all good.
 

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Here is the chart I have been going buy. I am know that all companies 37" tires are not true 37"...more like 35.5-36"...so with that in mind for my 2015 JKU Automatic currently with 4.10 gears... I was leaning towards 4.56 gears...still considering 4.88 but I have time to think this out as my tire swap will not happen until this winter at the earliest.
 

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My tires are 37 in Cooper STT pros and run about 36.25 tall I'm in a 2016 jku 4.88 is perfect. Some will tell you 5.13 but that would be overkill imo.


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I run 37s Trail Grapplers and they are pretty much true to size - 5.13 is perfect in an automatic (daily driver and rock crawler). Folks with Rubicons should also take note that being correctly geared is what really allows that 4:1 Transfercase to shine. Anything less than correct gearing in a Rubicon is wasting the heart of what makes a Rubicon a Rubicon.
 

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With a 3.6 and an auto? I'm up around 3k rpm at 75-80. Not that I should be going that fast..... :)


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Good feedback thanks...confusing though compared to the cart I posted. When I go on a 4 hour highway trip I drive 75mph....I certainly do not want to be at 3000 rpm at 75mph....
 
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