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Old 11-15-2012, 04:25 AM   #1
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Difference in gears, 3.21-3.73-4.10

If the drive train on our 2013's is the same in the Sport, what is the difference in having these gears?

Does the 4.10 run at a higher RPM on the highway than the 3.21's? I am at 2100 at 70 MPH.

Do you run differently in the 3.73 or 4.10?

Seems the 4.10's would be about 2500.........?

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Old 11-15-2012, 05:20 AM   #2
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I had an 82 Toyota pickup that I run 4.10 gears in... It is deff not made to run on the high way bc your top speed is only going to be around 55 mph... They will put a lot more torque to the rear tires....3.73 on the the other hand is made for the highway and is pretty good on fuel... I have 4.56 in my jeep but for the best on road/ off road gears I would recommend 4.88 ....


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Old 11-15-2012, 05:31 AM   #3
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Yes the 4.10 will turn more RPMS..


the ratio is 4.10 drive shaft turns to 1 revolution of the rear wheels.. or 3.73 drive shaft turns to 1 rear wheel revolution or 3.21 drive shaft turns to 1 rear wheel revolutions..
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Old 11-15-2012, 07:52 AM   #4
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You'll likely get better mileage with a 3.21. I usually use my 4.10 '12 JKUR as a DD - it reads around 17 on the freeway, my '12 3.21 JKU is a bit lighter, but reads 19-20. Both are running the same size Duratracs and speedo corrected. yes, the speedo is just over 2K with the 3.21s.
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Old 11-15-2012, 08:48 AM   #5
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The #1 reason to change gears is to offset the resistance found when increasing tire size. When you go to 35" tires, your workload on the Jeep increases tremendously. The higher gears become necessary to restore performance, and in some cases recover lost fuel economy.
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Old 11-15-2012, 09:12 AM   #6
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Here's a pretty good site. Not sure what the specs are on the 13's but if you can find all the pertinent numbers, you can do a side by side comparison.

Gear Ratio Calculator
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Old 11-15-2012, 09:38 AM   #7
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I have 4.10s and get around 17 to 18 mpg on highway AND surface streets. It runs about 2700 rpm at 70 mph. (Kinda sucks but ain't too bad) where it becomes amazing is offroad and uphill. I was behind my buddy and his are 3.21s . Every incline we hit his speed dropped DRAMATICALLY while I could have easily passed him and increased speed... I would definitely recommend if ur going to be off road alot... First gear and four low this thing walks over anything ... Not to mention when I flip the lockers on hehe
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Old 11-15-2012, 10:50 AM   #8
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i have 35's on 4.10's 2007 JKU 4D & get pretty big top end speed when i want it. hovers around 2krmp @ 70 (70 actual, not spedo, check with gps & add 1mph for latency). i'd imagine if i had smaller tires my top end speed would really suck due to high rpm. my advice, if you're going with 4.10 slap some 35's on there.
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Old 11-15-2012, 11:42 AM   #9
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Originally Posted by DRoberts
i have 35's on 4.10's 2007 JKU 4D & get pretty big top end speed when i want it. hovers around 2krmp @ 70 (70 actual, not spedo, check with gps & add 1mph for latency). i'd imagine if i had smaller tires my top end speed would really suck due to high rpm. my advice, if you're going with 4.10 slap some 35's on there.
Good to know for when mine arrive... Was worried about re gearing
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Old 11-15-2012, 08:07 PM   #10
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I have a '12 JK Sport with 3.21's running 285/70-17 Nitto Terra Grapplers (they measure 31.75" under the Jeep).

I just had them installed last week. The JK definitely feels more sluggish from a stop but it not too terribly bad.

I made a trip today from Pascagoula, MS to Cocoa Beach, FL - at 75 MPH I was turning 2,000 RPM in 6th gear. I kept the cruise control locked on for most of the trip and the Jeep was able to maintain speed with no issues. I only had to downshift once, when I had to slow to about 45-50 going up a hill. Slipped down to 5th until I got the speed up to 65 and no other issue.

I am not sure how my mileage is going to turn out, but I am monitoring and will compare to my trip here last month when I was running stock 225/75-16's.

I believe 4.10's may be too steep for my set-up and 3.73's may get me back to stock like feel.
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Old 12-02-2012, 06:40 PM   #11
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Originally Posted by LudlowJKUR View Post
I have 4.10s and get around 17 to 18 mpg on highway AND surface streets. It runs about 2700 rpm at 70 mph. (Kinda sucks but ain't too bad) where it becomes amazing is offroad and uphill. I was behind my buddy and his are 3.21s . Every incline we hit his speed dropped DRAMATICALLY while I could have easily passed him and increased speed... I would definitely recommend if ur going to be off road alot....hehe
Getting ready to order 2013 Sahara and was thinking 3.21 was the way to go for mpg as wife will use 90% as a mall crawler, BUT we also live at the bottom of a fairly steep hill and have to go up 2 or 3 times a day to get anywhere.

Are you saying the 3.73 option would offer better torque in getting up the hills or not that much difference compared to 3.21? I'm more about performance than mpg. Thanks.
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Old 12-02-2012, 06:48 PM   #12
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Getting ready to order 2013 Sahara and was thinking 3.21 was the way to go for mpg as wife will use 90% as a mall crawler, BUT we also live at the bottom of a fairly steep hill and have to go up 2 or 3 times a day to get anywhere.

Are you saying the 3.73 option would offer better torque in getting up the hills or not that much difference compared to 3.21? I'm more about performance than mpg. Thanks.
you answered your own question. 3.73's will out perform 3.21's all things being equal. That said I live in NW Montana where nothing is flat and I love my 3.21's but I don't have 35" tires either.
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Old 12-02-2012, 07:27 PM   #13
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Originally Posted by Rtone1583 View Post
Here's a pretty good site. Not sure what the specs are on the 13's but if you can find all the pertinent numbers, you can do a side by side comparison.

Gear Ratio Calculator
The guy who wrote that must be a genius.
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Old 12-02-2012, 08:18 PM   #14
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The worst part of 3.21 gears is when you do regear you have to buy new carriers for the diffs. No need if you are 3.73 going to 4.56 or other.
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Old 12-09-2012, 10:34 AM   #15
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Just test drove a 2013 3.21 and then a 3.73, both with factory wheels and tires. I felt a world of difference. The 3.21 was sluggish and slow, especially at the lower speeds. The 3.73 felt good and responsive. I'm ordering in the next couple of days and will definately go with the 3.73.\
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Old 12-09-2012, 11:13 AM   #16
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Quote:
Originally Posted by PapasJP View Post
If the drive train on our 2013's is the same in the Sport, what is the difference in having these gears?

Does the 4.10 run at a higher RPM on the highway than the 3.21's? I am at 2100 at 70 MPH.

Do you run differently in the 3.73 or 4.10?

Seems the 4.10's would be about 2500.........?
The difference in engine rpm's with different axle ratios is easy to figure. It's an exact mathematical difference. Example:

4.10 axle ratio compared to a 3.21 ratio

4.10 divided by 3.21 = 1.277. That means a 4.10 axle ratio will turn 27.7% more engine rpm's at any given vehicle speed.

or

3.21 divided by 4.10 = .7829. That means a 3.21 axle ratio will turn 78.29% fewer engine rpm's at any given vehicle speed

In your example above at 2100 rpm's with a 3.21 ratio, you would be running about 2682 rpm's with a 4.10 ratio.

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Old 12-09-2012, 01:01 PM   #17
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Originally Posted by LudlowJKUR View Post
I have 4.10s and get around 17 to 18 mpg on highway AND surface streets. It runs about 2700 rpm at 70 mph. (Kinda sucks but ain't too bad) where it becomes amazing is offroad and uphill. I was behind my buddy and his are 3.21s . Every incline we hit his speed dropped DRAMATICALLY while I could have easily passed him and increased speed... I would definitely recommend if ur going to be off road alot... First gear and four low this thing walks over anything ... Not to mention when I flip the lockers on hehe
This. I Traded in my 3.21 jku for a Rubi and I love the 4.10s.
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Old 12-09-2012, 01:03 PM   #18
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....3.21 divided by 4.10 = .7829. That means a 3.21 axle ratio will turn 78.29% fewer engine rpm's at any given vehicle speed ...
Minor correction - 3.21 axle ratio will turn 21.71% fewer engine rpm's at any given vehicle speed. Or 78.29% of the RPMs of a 4.10 axle.
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Old 12-09-2012, 01:46 PM   #19
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Dumb question; how do you know what gearing you have?
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Old 12-09-2012, 01:54 PM   #20
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Dumb question; how do you know what gearing you have?
Its not dumb I was thinking the same thing or were both dumb lol
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Old 12-09-2012, 01:57 PM   #21
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Nor sure what gears i have but at 70mph im at around 2900rpms with 35" nitto trail grapplers. Cold air intake and throttle body spacer and superchips programer. Not sure how to use programer
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Old 12-09-2012, 02:01 PM   #22
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Dumb question; how do you know what gearing you have?
an easy way is just jack up your rig and have some one hand spin your tire 1 full revelution and count how many times the drive shaft turns or just have a jeep service dept run your vin and they can tell you.
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Old 12-09-2012, 04:55 PM   #23
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an easy way is just jack up your rig and have some one hand spin your tire 1 full revelution and count how many times the drive shaft turns or just have a jeep service dept run your vin and they can tell you.
Well I found out what gearing I have by going onto the Mopar Owner Connect web site & then going to my Dashboard, then Learn & Use, then Equipment/Standard & there it was: 3.73 Rear Axle Ratio
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Old 12-09-2012, 05:30 PM   #24
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Minor correction - 3.21 axle ratio will turn 21.71% fewer engine rpm's at any given vehicle speed. Or 78.29% of the RPMs of a 4.10 axle.
Oops, my mistake.

You are correct. A 3.21 axle ratio will turn 21.71% fewer engine rpm's at any given vehicle speed, when compared to a 4.10 ratio.

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Old 02-06-2013, 12:02 PM   #25
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Just test drove a 2013 3.21 and then a 3.73, both with factory wheels and tires. I felt a world of difference. The 3.21 was sluggish and slow, especially at the lower speeds. The 3.73 felt good and responsive. I'm ordering in the next couple of days and will definately go with the 3.73.\
Do you think the 3.73 gears would compensate for the 4 doors addition weight vs. the 2 door in acceleration?
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Old 02-06-2013, 12:56 PM   #26
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One other aspect to consider - gear spacing. Steeper gears make your gear spacing "tighter" so that each individual gear is responsible for a narrower range of MPH. Take a quick hypothetical vehicle - 1:1 rear gear ratio and a 2 speed transmission, 2:1 first with an rpm-limited top speed of 50 mph and 1:1 second with an rpm-limited top speed of 100 mph. Change the rear gear ratio to 2:1 - top speed of 25 mph in first gear and 50 mph in second gear. With 1:1 gears, first and second gear are each "responsible" for 50 mph, with 2:1 gears, they are each "responsible" for 25 mph.

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