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Old 06-27-2015, 12:28 PM   #1
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Differential Ratio's needed, for different tire sizes

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...
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Old 06-27-2015, 02:32 PM   #2
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Quote:
Originally Posted by Old Dogger View Post
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...
I vote "Sticky" on this. Nice job.

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Old 09-26-2015, 01:50 PM   #4
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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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Old 09-28-2015, 11:06 AM   #5
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Quote:
Originally Posted by dynojoe View Post
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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Old 09-28-2015, 11:25 AM   #6
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Quote:
Originally Posted by dynojoe View Post
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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Old 09-28-2015, 12:01 PM   #7
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Old 07-03-2016, 10:47 PM   #8
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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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Old 07-04-2016, 01:25 AM   #9
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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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Old 07-04-2016, 02:25 AM   #10
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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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Old 07-04-2016, 02:22 PM   #11
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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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Old 07-04-2016, 02:52 PM   #12
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Since you have the HR, which is somewhat heavier, 4.88 Ratio..IMO
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Old 07-04-2016, 10:54 PM   #13
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Quote:
Originally Posted by mdrums View Post
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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Old 07-04-2016, 11:03 PM   #14
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Waste of money to go from 4:10 to 4:56 .Go with 4:88 if you know you are going to 37s.

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Old 07-04-2016, 11:31 PM   #15
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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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Old 07-06-2016, 08:57 AM   #16
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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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Old 07-06-2016, 09:01 AM   #17
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Differential Ratio's needed, for different tire sizes

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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Old 07-06-2016, 10:04 AM   #18
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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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Old 07-06-2016, 10:08 AM   #19
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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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Old 07-06-2016, 10:19 AM   #20
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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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Old 07-06-2016, 10:24 AM   #21
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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....
This is actually the most efficient power band for the 3.6 engine.
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Old 07-06-2016, 10:25 AM   #22
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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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Yep. This is actually the prime operating band for the 3.6. I run a bit over 2700 at 70.
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Old 07-06-2016, 11:29 AM   #23
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Yep. This is actually the prime operating band for the 3.6. I run a bit over 2700 at 70.


My mechanic had the 5.13 with 37's and said rpm was consistently high on the highway. He since stepped down to 4.88. If you look at the chart running around a 36-37 tire will put you only slightly above stock RPM or running 5.13 will put you over stock rpm. And most of us have 37's closer to 36. So if one would be happy with slightly better acceleration pickup over stock 4.88 would actually be the solid choice. If you are more only for offroad and don't mind higher over stock rpms on highway then 5.13's. But with the finer teeth of 5.13 you are at a higher chance to strip a gear. When on one of the trails this weekend I climbed a extremely high angle wall no problem with 4.88. Can't imagine ever wanting more. Just my 2 cents.


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Old 07-06-2016, 11:40 AM   #24
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Stock rubicon wheels run about 32 inches tall

32's with 3.73 gears 2276 RPM at 70

32's with 4.1 gears. 2502 RPM at 70

36 with 4.88 gears 2647 RPM at 70

36 with 5.13 gears 2783 RPM at 70

37's with 4.88 gears. 2576 RPM at 70

37's with 5.13 gears 2708 RPM at 70


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Old 07-06-2016, 11:58 AM   #25
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Quote:
Originally Posted by bengr View Post
My mechanic had the 5.13 with 37's and said rpm was consistently high on the highway. He since stepped down to 4.88. If you look at the chart running around a 36-37 tire will put you only slightly above stock RPM or running 5.13 will put you over stock rpm. And most of us have 37's closer to 36. So if one would be happy with slightly better acceleration pickup over stock 4.88 would actually be the solid choice. If you are more only for offroad and don't mind higher over stock rpms on highway then 5.13's. But with the finer teeth of 5.13 you are at a higher chance to strip a gear. When on one of the trails this weekend I climbed a extremely high angle wall no problem with 4.88. Can't imagine ever wanting more. Just my 2 cents.


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I would not go down to 4.88 myself but I am about a true 37. Again, this engine likes the higher rpm range. It's where it works its best. Solution to not stripping gears is knowing how to wheel - and when crawling lower gears actually help this because you have less wheel spin (which is often when things begin to go boom when you get wheel spin). That's why they call it rock crawling - the lower you can crawl, the better it is on obstacles for your rig.
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Old 07-06-2016, 12:26 PM   #26
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Quote:
Originally Posted by Old Dogger View Post
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...
I think people also have to take into account if the Jeep is a daily driver, if so you don't want to run gears too big! Yes you'll get more power off the line to get up rocks and through mud, but you will get really shitty gas mileage while driving to work etc.
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Old 07-06-2016, 12:43 PM   #27
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4 door running heavy (4800) manual trans, tows boat and will take to the Smokies. 315/70/17 duratracs. Was considering 4.56 but I start doing some calculations and 4.88 would take me to 3000 rpm at 80 mph. 80 mph is as fast as I will take this Jeep, there are just not many places around here for that, one of them is toward the hills. My ride to work is 16 miles, 60 mph max, and I rarely put it in 6th. Currently running 3.73 which yes is doable but certainly not optimal. I think if I had auto I would go 4.56
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Old 07-06-2016, 12:50 PM   #28
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I checked mine this morning - GPS verified speed of 70mph and in high gear I'm running exactly 2000 rpm. 2015 2-door JK, auto, 3.21 gears, 305/65-17 (32.4") tires.
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Old 07-06-2016, 01:12 PM   #29
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Originally Posted by EricZ_ View Post
I think people also have to take into account if the Jeep is a daily driver, if so you don't want to run gears too big! Yes you'll get more power off the line to get up rocks and through mud, but you will get really shitty gas mileage while driving to work etc.
Proper gearing is not just about gas mileage. A mainly street rig not doing difficult stiff offroad should aim for the green colored blocks in the charts above.
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Old 07-06-2016, 06:27 PM   #30
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Originally Posted by mommymallcrawler View Post
Quote:
Originally Posted by EricZ_ View Post
I think people also have to take into account if the Jeep is a daily driver, if so you don't want to run gears too big! Yes you'll get more power off the line to get up rocks and through mud, but you will get really shitty gas mileage while driving to work etc.
Proper gearing is not just about gas mileage. A mainly street rig not doing difficult stiff offroad should aim for the green colored blocks in the charts above.
So if was looking to regear my Sahara I currently have 3.21 stock gears. Should I go to 3.73? Or just do 4.21?
I have 35's and I go crawling every chance I get. But it's also a daily driver.

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