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Old 12-01-2009, 11:26 AM   #1
btircuit
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Which Gears 4.88 or 5.13

Took the first highway drive this weekend since the 35's were installed. It drove fine as long I kept it around 60-65 but as soon as I pushed it to 70 it started a down shifting cycle every 20 seconds or so. No way I can live with it this way... gotta get new gears.
Now the question is 4.88 or 5.13's? I really don't see myself going any larger than 35's so I'm leaning toward 4.88.
What do you guys think?
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Old 12-01-2009, 12:29 PM   #2
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5.13

have you used a procal or something to adjust the speedo (tire size)? I did it after installing 35s and it helped the gear search
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Old 12-01-2009, 12:41 PM   #3
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Yeah, did Pro Cal speedo and tire size so I figured it's down to re-gearing.
I'm with ya... 5.13's. Do them once and forget it.
Thanks
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Old 12-01-2009, 03:43 PM   #4
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Let me be the contrarian here, I had to make the same decision. Being of the opinion I'd never go beyond 35s on a JK with stock axles, I opted for 4.88s. The reason being that at cruising speed the 4.88s will have the RPMs at just over 2k, just about perfect. Using the AEV module to recal for 35" KM2s I've had no issues whatsoever with OD hunting, etc., and in low range it crawls great. JMHO, of course.
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Old 12-01-2009, 04:06 PM   #5
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things to consider:

Rubi? you have 4.10 and going to 4.88 would be worthless and a waste of money

Daily driver? trail rig? daily driver/andtrail rig?

auto? stick?

35s now but ever going bigger?

no matter what you answer I would go with at least 5.13
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Old 12-01-2009, 05:18 PM   #6
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I just did 5.13's w/37's. 6-sp. 1st gear is actually useful now. 6th gear at 60 mph (I did use the procal to adjust the speedo) is about 2400 rpm's, seems a bit high to me. I wonder what 5.13's and 35's would be?? Fuel economy has gotta suck... but since I don't keep track of mine and really don't care anyway, I don't really know.
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Old 12-01-2009, 05:19 PM   #7
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2400 is fine..... these litle hamster wheels like high rpms
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Old 12-01-2009, 08:55 PM   #8
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It's a Rubicon Auto 4.10's pulled behind an RV used mostly for trail riding.

A friend has the same setup with 4.88's and has been somewhat happy but says he'd like to go to 5.13's. But he's also considering 37's which I"m not.

Coin flip...
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Old 12-02-2009, 06:17 AM   #9
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There is a TON of info on here regarding this very issue. It took me a few minutes but I dug up a good post.

Although most of this info pertains to the manual, this is a good read as it gives you some perspective on the JK auto tranny numbers after a regear.
http://www.jkowners.com/forum/showth...s+rubi+highway

Xtremjeepn made an excellent point in that thread that you should consider:
The Auto has an INSANELY deep overdrive compared to the stick. Run this number. A Rubi, 4:10 6 speed with 35s rolling down the highway is 40 rpm(yes, 40) less than an Auto with 5:13s and 35s.

So, you'd need to go to at least 5:13's to bring it back to stock. Many have even gone to 5:38's. Once you get too deep you need to consider pinion size and contact patch. I don't know the reliability of the 5:38's on the stock d44's but from what I've read on here people are having good luck so far. If you're mostly trail riding the deeper gears are obviously a better choice for you. Furthermore, since you'll add the heavier tires/wheels you're now making that engine work even harder. This doesn't even factor in the armor and other stuff you pack in the JK when heading down the trail. If you weigh your JK once it's loaded down, you'll find that it's around 5.5K to 6K lbs.

Make sure you read this thread too. It's got some good numbers on it. http://www.jkowners.com/forum/showth...ght=5%3A38%27s

GOOD LUCK and I hope this helps.


Tim
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Old 12-02-2009, 06:50 AM   #10
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Quote:
Originally Posted by btircuit View Post
It's a Rubicon Auto 4.10's pulled behind an RV used mostly for trail riding.
auto w/35s = 5.13 NO DOUBT
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Old 12-02-2009, 08:31 AM   #11
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Very Helpful

Thanks for the links... great food for thought.
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Old 12-02-2009, 08:36 AM   #12
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5.13's

I'm on my 4th or 5th JK in the shop and any combination of tranny and tires that were 35+ I always wished I would've gone lower than that 4.88's in the ones I regeared to 4.88
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Old 12-02-2009, 11:01 AM   #13
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Thats funny I was just thinking the same thing I am getting 35s and will be regearing in feb or march but didnt know what I wanted to go to. So I am going to watch the thread
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Old 12-02-2009, 02:14 PM   #14
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If you have the Dana 30 in the front I would stay with 4.88. If you play hard you will chew up the 5.13 ring gear. I went through a few.I was fine just playing on the trail rides but when I started to run it in the comps. You play a little harder and then you get that oh so familiar sound of teeth stripping.
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Old 12-03-2009, 05:35 PM   #15
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I have never seen a post about properly installed gears on a JK failing with 37" or smaller tires unless it was part of a catastrophic failure that would have damaged any size gears.

So, the evidence would suggest that the gearset is not the weak link in JK axles and you shouldn't worry about putting 5.13s in a JK Dana 30, or 5.38s in a JK Dana 44. If you have seen a compelling number of JK gearset failures due to small gear teeth, please point them at least one out to me on any forum.

As a result, with an automatic, I'd go absolutely as low as you can go. For a Rubi, that is 5.38s, for a non-Rubi, that is 5.13s.

The JK engines require higher rpm levels than the old 4.0L TJ engines. They really need to be run over 2500 rpm at highway speeds to produce enough rear wheel horsepower to drive in windy situations, towing, up grades, or at higher elevations to avoid flashing the converter (downshifting with tranny slip that revs the rpm levels higher than they would otherwise be in just 3rd gear with the overdrive off). I am not an engineer or a rocket scientist, but I can tell you that 64 rear wheel horsepower at 2k rpm is not adequate to roll 35" tires on a non-aerodynamic brick of a lifted JK in windy, towing, graded/hilly, or higher elevation (i.e. Rocky Mountain West) situation. Maybe on low elevation, flat highways, on 2 doors, with little or no lift 64 rear wheel horspower at 2k rpm is sufficient, but not for the rest of us.

Gas mileage will improve for automatics that run at 2500 rpm at highway speeds due to the reduced flashing of the converter (downshifting, etc.).

This is not opinion or casual observation. Clearly, the automatic JKs are undergeared stock and the Rubi's should have come with 4.56s or 4.88s stock.

Here is the evidence of the JK rear wheel horspower at different rpm levels:





Here are the rpm levels by tranny type, gear, and tire size (notice that the rpm levels of 5.38s are barely more than 5.13s, making 5.38s preferable with an automatic Rubi with 35" or taller tires):

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Old 12-03-2009, 05:55 PM   #16
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Good post.
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Old 12-03-2009, 06:32 PM   #17
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Ok so which brand would be the best to go with? Alloy usa , motive gear or the g2?
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Old 12-15-2009, 06:50 PM   #18
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Anyone got a good choice or best brand of gears to go with?
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Old 12-15-2009, 09:44 PM   #19
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I would just call one of the supporting vendors here and ask their opinion--probably Phil (owns Trail Duty).
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Old 12-15-2009, 09:50 PM   #20
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I went with the Yukon gears, Im sure others are ok as well. most are made by the same manufacter and relabled before sale. but to be safe. Go with Yukon.
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Old 01-10-2010, 08:41 AM   #21
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Would GR apply to me?

Quote:
Originally Posted by planman View Post
I have never seen a post about properly installed gears on a JK failing with 37" or smaller tires unless it was part of a catastrophic failure that would have damaged any size gears.

So, the evidence would suggest that the gearset is not the weak link in JK axles and you shouldn't worry about putting 5.13s in a JK Dana 30, or 5.38s in a JK Dana 44. If you have seen a compelling number of JK gearset failures due to small gear teeth, please point them at least one out to me on any forum.

As a result, with an automatic, I'd go absolutely as low as you can go. For a Rubi, that is 5.38s, for a non-Rubi, that is 5.13s.

The JK engines require higher rpm levels than the old 4.0L TJ engines. They really need to be run over 2500 rpm at highway speeds to produce enough rear wheel horsepower to drive in windy situations, towing, up grades, or at higher elevations to avoid flashing the converter (downshifting with tranny slip that revs the rpm levels higher than they would otherwise be in just 3rd gear with the overdrive off). I am not an engineer or a rocket scientist, but I can tell you that 64 rear wheel horsepower at 2k rpm is not adequate to roll 35" tires on a non-aerodynamic brick of a lifted JK in windy, towing, graded/hilly, or higher elevation (i.e. Rocky Mountain West) situation. Maybe on low elevation, flat highways, on 2 doors, with little or no lift 64 rear wheel horspower at 2k rpm is sufficient, but not for the rest of us.

Gas mileage will improve for automatics that run at 2500 rpm at highway speeds due to the reduced flashing of the converter (downshifting, etc.).

This is not opinion or casual observation. Clearly, the automatic JKs are undergeared stock and the Rubi's should have come with 4.56s or 4.88s stock.

Here is the evidence of the JK rear wheel horspower at different rpm levels:





Here are the rpm levels by tranny type, gear, and tire size (notice that the rpm levels of 5.38s are barely more than 5.13s, making 5.38s preferable with an automatic Rubi with 35" or taller tires):

I have a 6spd 06 Rubi with 4.0. I use the thing a lot on highway @ 70-75 mph. I hate seeing the engine rev so high. In 6th I tach 2700 @ 70mph. Can I use this chart to select a larger tire to stay in the green range yet give the engine a break? My 4.0 91 Wrangler (5spd) was in the 2200 range at these speeds if I remember correctly and that engine was running like a top when I sold the Jeep @ 197,000 miles. Great post by the way.

Bob D
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Old 01-10-2010, 09:43 AM   #22
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So alot of people say since this is my daily driver 4.88s would be the way to go but then othrs say all the weight you add you should do 5.13s damn such a hard choice
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Old 01-10-2010, 10:15 AM   #23
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I am trying to make the same choice and honestly after talking with some people and riding in a guys with the 5.13 gears I am now set at 5.13's and actually if I had the front dana 44 I would be doing 5.38's

My Jeep is a daily driver but I think that the 4.88's will fall short of some of my expectations and the difference between the 4.10's and the 4.88's does not really backup the price as the 5.13's do.
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Old 01-10-2010, 11:35 AM   #24
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they should put 4.88s in stock. go with the 5.13s.

i have 5.38s in and love them.
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Old 01-10-2010, 11:41 AM   #25
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i have the been wondering about the gear manufacturers. i hear about noisy gears with some brands. i understand yukon is made in china. is there a quality American made set of gears available? i would like to run a 4.56 with my 6 speed as i don't think i will go over a 35 inch tire, let alone allot of advertised 35 inch tires are more like 34"

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