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Old 10-25-2013, 07:27 PM   #1
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GEARS?? 35's and 37's in the future....

2013 Jeep Unlimited Sahara, automatic, 3.21s

My questions is what gears should I run that will be good with 35's and 37's?

I am installing 35's with a 3.5inch AEV lift. I have 3.21's and will need to change the gears. My Jeep will be a Daily Driver and will only see the trails a few times a year.

Please help me out here guys.

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Old 10-25-2013, 07:36 PM   #2
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From all the other threads on this I think they start at 4.56s and get steeper from there.

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Old 10-25-2013, 07:41 PM   #3
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I would say 4.56 are pretty tall for every day use. I have 3.73s on my 35s and it runs at good rpms on freeway and daily driving
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Old 10-25-2013, 07:43 PM   #4
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4:56 will be great for 37s 4:10s would be better for 35s.
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Old 10-25-2013, 07:45 PM   #5
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From all the other threads on this I think they start at 4.56s and get steeper from there.
I keep hearing 4.56, would 4.88 be to much?
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Old 10-25-2013, 07:47 PM   #6
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4:56 will be great for 37s 4:10s would be better for 35s.
If I upgraded to 37's down the road will 4.10's still be good?
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Old 10-25-2013, 08:14 PM   #7
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No I would want 4:56 for 37s.
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Old 10-25-2013, 08:24 PM   #8
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No I would want 4:56 for 37s.
I agree, or maybe 4.88s. I have 4.10s with 35s and just changed to 37s. The 4.10s were ok, with the new tires I really need a shorter gear, will probably go 4.88s
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Old 10-25-2013, 08:29 PM   #9
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Originally Posted by Blackonwhite View Post
2013 Jeep Unlimited Sahara, automatic, 3.21s

My questions is what gears should I run that will be good with 35's and 37's?

I am installing 35's with a 3.5inch AEV lift. I have 3.21's and will need to change the gears. My Jeep will be a Daily Driver and will only see the trails a few times a year.

Please help me out here guys.


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Old 10-25-2013, 08:42 PM   #10
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Wow! That chart says I'll turn 2425rpm with my 37s and my 4.10 gears. Maybe at 90mph! At 60 I turn 2000.
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Old 10-25-2013, 08:43 PM   #11
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I think it depends a lot on which brand 35 or 37" you go with as well, because many don't measure out to their manufacturer's size once under load and if you're running less than manufacturer's recommended psi (My 35's recommend 50psi max, I'm running 28 psi).

For example I just bought 35x12.5x17" BFG KM2s and measured them to program the speedo with the procal and they measured out to be 33.5" almost exactly.

I don't notice a big difference in performance, however I do noticed more in braking and in obviously side to side (lane changes, etc) movements.

I'm happy with my tires with the 4.10's on the Rubi, but I can't help but wonder how much 37" KM2's measure out to in the real world, because already I'm thinking I'd like a bit bigger, and these tires are about 3 days old! If the 37's measure out to 35.5" I'm sure they would be tolerable as well with 4.10's. Also installed them pre-lift with no rub at all on the street with no rubi rails or front airdam. No rub even at full lock which surprised me.
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Old 10-25-2013, 08:45 PM   #12
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Wow! That chart says I'll turn 2425rpm with my 37s and my 4.10 gears. Maybe at 90mph! At 60 I turn 2000.

60 at 2000 rpm.. is that in OD in Auto? What brand 37's do you own and what do they measure out at ?
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Old 10-25-2013, 08:59 PM   #13
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I think it depends a lot on which brand 35 or 37" you go with as well, because many don't measure out to their manufacturer's size once under load and if you're running less than manufacturer's recommended psi (My 35's recommend 50psi max, I'm running 28 psi).

For example I just bought 35x12.5x17" BFG KM2s and measured them to program the speedo with the procal and they measured out to be 33.5" almost exactly.

I don't notice a big difference in performance, however I do noticed more in braking and in obviously side to side (lane changes, etc) movements.

I'm happy with my tires with the 4.10's on the Rubi, but I can't help but wonder how much 37" KM2's measure out to in the real world, because already I'm thinking I'd like a bit bigger, and these tires are about 3 days old! If the 37's measure out to 35.5" I'm sure they would be tolerable as well with 4.10's. Also installed them pre-lift with no rub at all on the street with no rubi rails or front airdam. No rub even at full lock which surprised me.
I am thinking about running 35x12.50x17 Toyo MT's. I have had really good luck running Toyo's on lifted trucks. The MFG's are also good. I think the toyo's will have less road noise.
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Old 10-25-2013, 09:03 PM   #14
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60 at 2000 rpm.. is that in OD in Auto? What brand 37's do you own and what do they measure out at ?
Yes in OD, Nitto Trail Graplers, measure out to 36 1/4. The other chart I have always used has floated around here for quite some time and says I should turn 2164 at 70. Pretty much right on.

What speed does the above chart use? I'm guessing it also assumes 1:1 trans drive ratio so no OD.
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Old 10-25-2013, 10:15 PM   #15
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I agree, or maybe 4.88s. I have 4.10s with 35s and just changed to 37s. The 4.10s were ok, with the new tires I really need a shorter gear, will probably go 4.88s
Do you think 4.56 would be good with 35s and still good when I upgrade to 37s later
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Old 10-25-2013, 11:01 PM   #16
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Do you think 4.56 would be good with 35s and still good when I upgrade to 37s later
On a '12 + with auto trans, yes.
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Old 10-25-2013, 11:39 PM   #17
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On a '12 + with auto trans, yes.
Ok, any guess on fuel economy??
35s and 37s????
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Old 10-25-2013, 11:40 PM   #18
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I would say 4.56 are pretty tall for every day use. I have 3.73s on my 35s and it runs at good rpms on freeway and daily driving
a

What kind of fuel economy do you get??
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Old 10-25-2013, 11:42 PM   #19
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I agree, or maybe 4.88s. I have 4.10s with 35s and just changed to 37s. The 4.10s were ok, with the new tires I really need a shorter gear, will probably go 4.88s
What is your MPG?
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Old 10-26-2013, 02:12 AM   #20
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On a '12 + with auto trans, yes.
Right. 12+ trans is geared WAY lower: http://www.wranglerforum.com/f33/no-...ml#post5490826
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Old 10-26-2013, 02:18 AM   #21
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i would say 4.56 are pretty tall for every day use. I have 3.73s on my 35s and it runs at good rpms on freeway and daily driving
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Old 10-26-2013, 02:30 AM   #22
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I have 35 KM2's on my 13 auto JKUR. I had 3.73's in it and it ran great on the street and freeway. I got 18 or 19 MPG freeway 16 in town. Now I have 4.56 and I have to drive 65 to stay under 2500. Over that the mileage drops from 20 to 18. Around town I now get 15. If mine was my daily driver I would never put 4.56 in it. As a trail rig that sees hard trails once a month yes. 3.73's with a set of Duratracs would be a much better choice. They roll easier, their lighter, and they work on easy trails. The KM2s are noisy. Ill go back to MTRKs next time. But in a 37 with the 4.56.

The new 580 trans has a lower first gear and a higher 5th gear. .89 vs .69 on the 42 RLE trans. You cant run the gears you did on the early trans you'll wind it out on the freeway and fuel economy will suffer if you go over 75. I know cause mine does.
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Old 10-26-2013, 09:13 AM   #23
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I have 35 KM2's on my 13 auto JKUR. I had 3.73's in it and it ran great on the street and freeway. I got 18 or 19 MPG freeway 16 in town. Now I have 4.56 and I have to drive 65 to stay under 2500. Over that the mileage drops from 20 to 18. Around town I now get 15. If mine was my daily driver I would never put 4.56 in it. As a trail rig that sees hard trails once a month yes. 3.73's with a set of Duratracs would be a much better choice. They roll easier, their lighter, and they work on easy trails. The KM2s are noisy. Ill go back to MTRKs next time. But in a 37 with the 4.56.

The new 580 trans has a lower first gear and a higher 5th gear. .89 vs .69 on the 42 RLE trans. You cant run the gears you did on the early trans you'll wind it out on the freeway and fuel economy will suffer if you go over 75. I know cause mine does.
First gear is lower, fifth is lower than the old fourth too. That is why the 580 turns more rpms on the highway at a given speed.

Don't be afraid to spin that engine way over 2500 on the highway, they love rpms. I don't know or care much about fuel economy on my Jeep though, it is a toy vehicle and often sits weeks at a time. Thirteen months in I only have 3,000 miles on it but those miles are all Colorado and Utah miles. I trailer it from home, my SuperCrew is much more comfortable on the road.
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Old 10-26-2013, 09:17 AM   #24
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I've got the 3.73s with 35s, and I think it's about right. It's about right on the road, and great off-road, but perhaps only because I have the 4:1, rather than the much higher-geared 2.73 transfer case ratio.

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I have 35 KM2's on my 13 auto JKUR. I had 3.73's in it and it ran great on the street and freeway. I got 18 or 19 MPG freeway 16 in town. Now I have 4.56 and I have to drive 65 to stay under 2500. Over that the mileage drops from 20 to 18. Around town I now get 15. If mine was my daily driver I would never put 4.56 in it. As a trail rig that sees hard trails once a month yes. 3.73's with a set of Duratracs would be a much better choice. They roll easier, their lighter, and they work on easy trails. The KM2s are noisy. Ill go back to MTRKs next time. But in a 37 with the 4.56.

The new 580 trans has a lower first gear and a higher 5th gear. .89 vs .69 on the 42 RLE trans. You cant run the gears you did on the early trans you'll wind it out on the freeway and fuel economy will suffer if you go over 75. I know cause mine does.
The new trans is lower in first gear (by 26%), but it also is 20% lower in 5th gear than the pre-'12 trans was in fourth (its highest) gear (0.89:1 is a lower, not higher, gearing than 0.69:1). To avoid a bunch of unnecessary typing, I regurgitate myself:

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Originally Posted by MarineHawk View Post
I didn't read through all 10 pages of this apparently meandering thread, but in case it's not covered: The appropriateness of 3.73s on your Jeep depends, in part, on the year of the JK.

2012+ Auto JKs have a 1st-gear ratio that is 26.4% lower than the 1st-gear ratio on the 2011 JKs (3.59:1 vs. 2.84:1).

That means (forgetting any Pentastar difference) that, in 1st gear, the 2012 JK Auto with 3.73 gears is geared as low as a 2011 with 4.71:1 gearing if such a thing existed (i.e., lower than 4.56 gears and getting close to 4.88s).

Interestingly, the 2012+ Auto gearing remains lower across the spectrum. For example, the top gear on the 2012+ JK (5th gear) is 20.3% lower than the top gear on the 2011 JK (4th gear) (0.83:1 vs. 0.69:1).

So, a 2012+ JK with 3.73:1 gears is geared like a pre-2012 JK with something like 4.56:1 gears.

Not sure if people realize that the Pentastar might not be the main difference between the pre-2012 JKs than the current models.
http://www.wranglerforum.com/f33/no-...ml#post5490826

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[I]f you have an after market transfer case or, in my case a Rubi, that can matter for the offroad needs. My transfer case drops the gearing by a factor of 4:1 in low range. That's a 46.5% difference over the 2.73:1 normal transfer case low-range ratio. So, if my 2012 Rubi with 3.73s is geared like a pre-2012 JK with 4.56:1 gears (it is). It's like me having 6.68 gears (crazy) in 4-low compared to a pre-2012 non-Rubi with a 2.73:1 transfer case in 4-low. It's definitely really low in 4-low. There are a lot of variables, but the 2012+ JKs are geared a lot lower than the pre-2012 JKs. And low enough for off-roading in 4-low unless you have really gargantuan tires.

The much lower gearing on the 5-speed 2012+ transmission makes a big difference across the board--forgetting even entirely the engine differences.
http://www.wranglerforum.com/f33/no-...ml#post5502354

Whether a regearing is required for a JK depends A LOT, for the on-pavement applications, on whether you have the new lower '12+ trans, or the much-higher-geared pre-'12 trans. And for many he off-road applications, also whether you have the Rubi's 4:1 transfer case drop.
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Old 10-26-2013, 10:02 AM   #25
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Here's my input.
Stock 2013 JKUR 10th Anniversary on 265/70/17 BFG KM2 which measure out to 31.7" using 4.10 gear ratio on the auto trans - 75% city/25% highway was getting 15mpg give or take 1/2 mpg.

Rugged Ridge Rims 35/12.5R17 Toyo Open Country ATII which measure out to be 34.5" also using the 4.10 gear ratio and exactly the same driving demands....

drum roll please 1st tank 15.4mpg!!!!!!!!!!!!

So 3" taller and 25 or so lbs heavier and I am getting the same or better gas mileage on 35's

I will say I had to go faster then I ever drive like 85mph or something to get the tach to go leave the "green" area on the RPM range (forget what numbers that is) but before the tire change I could go 72mph at the very top of the green range, now I can go 85. If I increased another 2" I would assume another 8 or so mph faster so 93mph...which I'm fairly certain I won't be using.

All I know is that with 35's the 4.10's are so ideal I am actually getting better gas mileage then with stock tires! With 37's I would imagine you would be back in the same situation I was with the 32's on 4.10's so 37's would need a 4:56 or lower gear ratio to really be ideal.

edit: I flash cal adjusted the diameter and yes I understand the difference between MT's and AT's with regard to tread blocking and compound.
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Old 10-26-2013, 10:17 AM   #26
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4:10s are the perfect ratio for 35s. 4:56 is more suited for 37s.
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Old 10-26-2013, 10:23 AM   #27
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[QUOTE="MarineHawk;5663529"]I've got the 3.73s with 35s, and I think it's about right. It's about right on the road, and great off-road, but perhaps only because I have the 4:1, rather than the much higher-geared 2.73 transfer case ratio. The new trans is lower in first gear (by 26%), but it also is 20% lower in 5th gear than the pre-'12 trans was in fourth (its highest) gear (0.89:1 is a lower, not higher, gearing than 0.69:1). To avoid a bunch of unnecessary typing, I regurgitate myself: [URL="http://www.wranglerforum.com/f33/no-3-73s-for-33s-no-73s-for-33s-no-no-no-329113-10.html#post5502354"]http://www.wranglerforum.com/f33/no-3-73s-for-33s-no-no-no-Whether a regearing is required for a JK depends A LOT, for the on-pavement applications, on whether you have the new lower '12+ trans, or the much-higher-geared pre-'12 trans. And for many he off-road applications, also whether you have the Rubi's 4:1 transfer case drop.

Wow, I am still racking my brain on exactly I should do. I have a 2013 unlimited automatic with 3.21 gears. I am installing a 3.5 AEV lift with toyo mt 35s. I was told by the shop to run 4.56 gears but from hearing you guys it seems I should only need 3.73. I will have to keep researching and calling around.
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Old 10-26-2013, 10:28 AM   #28
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[QUOTE=Blackonwhite;5664617]
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Originally Posted by MarineHawk View Post
Wow, I am still racking my brain on exactly I should do. I have a 2013 unlimited automatic with 3.21 gears. I am installing a 3.5 AEV lift with toyo mt 35s. I was told by the shop to run 4.56 gears but from hearing you guys it seems I should only need 3.73. I will have to keep researching and calling around.
I would say 4.10's are better unless you drive 70-80% highway and even then when in OD you will be running pretty low RPM and will have to drop a gear to do pretty much any speed increase.
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Old 10-26-2013, 11:06 AM   #29
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[QUOTE=Doctorsti;5664729]
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I would say 4.10's are better unless you drive 70-80% highway and even then when in OD you will be running pretty low RPM and will have to drop a gear to do pretty much any speed increase.
Now, if I run 4.10 and later upgrade to 37s will the 4.10 still be good?
Or is there somewhere in the middle that would be good for 35s or 37s.
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Old 10-26-2013, 01:32 PM   #30
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4:10s are the perfect ratio for 35s. 4:56 is more suited for 37s.
I disagree. I have both 35s and 4.10s, although I do have a manual. It's great for flats but any incline, towing, altitude change and its not ideal. I'm in Denver at 5,280 feet and its at the cusp of being under geared, especially with lots of bumper, armor, and winch weight.

So, I guess location, usage and mods are other factors that play into the formula

I'm really thinking 4.56 for a manual is going to be the magic number. No idea on the auto, which I realize is the primary content of this thread

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