[mgbug]

Would GR apply to me?

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

 

[steves07jk]

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

 

[d'sjk]

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.

 

[tank]

they should put 4.88s in stock. go with the 5.13s.

i have 5.38s in and love them.

 

[bubba2u]

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"

 

[TEXASKEV]

"is there a quality American made set of gears available?"

X2

 

[Hardcoreheath]

I got 5.13 installed, running an auto. I was turning about 3100 at 80, and turning about 2800 at 70, with 33' Kumho Road Venture MTs that are rated from the maufacturer actually closer to a 32". The around town was perfect, but the highway was maybe a tad too much rpm. And I mean just a tad. I just put on a set of Goodyear Wrangler MTRs today that are a 35x12.5x15. I havent had a chance to properly evaluate the highway manners, but so far in the city I wish I had gone to a 5.23 with the 35s. I think that would have made it a better overall arrangement then 33s and the 5.13s. However keep in mind my 33s were a bit on the short side compared to other brands. I am also running the Superchips Flashpaq on teh 87 octane performance tune.

 

[RPPR]

I have 3.73 auto unlimited. I towed a 6x12 trailer up and down some hilly highway and I agree with the mid-range RPMs. I had overdrive off with 35" toyo's going avg of 70 mph. My RPMs were around 27K sometimes 31K. It ran like a champ. At first I was thinking "too high, too high" heck no. If it had to down shift it would go to about 41K but at 27-32 it was a champ. easy to maintain speed and heck, there were times I was passing cars on both freeway hills and highway slopes. If I remember the trailer was around 3400 to 3700lbs

 

[ZBoater]

I've been reading a lot of these threads, and IF I were to regear, I'd do 4.88. Highway / daily driving is my primary concern. I don't tow, I don't spend my time driving up hills or against the wind, and even at 1900rpm I have yet to see a downshift. I've seen the speed drop 3-4 mph, but that's it. 2300 rpm would be my sweet spot. But being that the difference between that and my 4.10 I'd not that much, my brand new set of half doors is ahead of new gears in my mod roadmap...

 

[yellow rubicon]

gears

low is never enough; who gives a s...t about daily driving;it's a jeep not a honda civic;did you buy a jk to go on the highway,well you are a soccer mom then,I bought mine to wheel it!!!! I use my wife civic to go shopping or on the highway.So go with 5.38 and go crawl so rocks you won't regret it

 

[AikiJuan]

Been thinking about this topic for quite some time now and I haven't read one post regretting going with 5.13's. I hope I won't be the first I'm taking "Connie" in for regearing on Monday morning:bounce::bounce:. Sierra 5.13's very affordable and couldn't find anything bad written about them. Wish me luck!

 

[ZBoater]

low is never enough; who gives a s...t about daily driving;it's a jeep not a honda civic;did you buy a jk to go on the highway,well you are a soccer mom then,I bought mine to wheel it!!!! I use my wife civic to go shopping or on the highway.So go with 5.38 and go crawl so rocks you won't regret it

And THAT is why they make different gears. Different strokes for different folks...

 

[Hardcoreheath]

5.13s are not enough gear for 35s IMO. So far I'm not at all as happy as I was with 33s. Before I could go up any incline the Texas and Louisiana interstates had to throw at me, with full cargo and five passengers. Now I'm back to constantly downshifting to maintain 75mph on even small in lines. I'd be suicidal if I had put 4.88s in my junk. I'm so sick I'm contemplating going back to a 33-34 inch tire. Do not fear the gear. Do the math, you are talking a tiny bit more rpm to make a world of difference.

 

[jeepnick]

We Just Installed g2 gears on my 09 rubi, no noise
Install wasn't bad and it's the perfect amount of gears for 37s
I'm turning 3000rpms at 70
Would rather that be alittle less but oh well
Btw it's a manual

 

[AikiJuan]

Ok guys just dropped off Connie for her gear swap to 5.13's at Roco 4X4. I'll let you know how she runs when I pick her up tomorrow. Hope all goes well.

 

[AikiJuan]

Finally got my 5.13's in and I've got to tell you that I'm shocked how well she runs now. I'm really glad I didn't go 4.88 like I was planning to do. It runs really good on the highway too. I can't wait for the break-in so I can pull the camper and see how it does. I expect it will be just fine. Depending on how much highway daily driving you do you could even go 5.38's. By the way if you are local and are looking for a good shop to get work done give Roco 4x4 a call.

 

[randyb]

I had 513"s installed last week on my 2010 Rubi. I run 35"s. This by far is the best thing I have done since I have owned the Jeep. After I lifted it and put on the 35"s it sucked on the freeway. Now it's great. I drove it last night to the bowling alley head on into the winds (15 mph or so) we are getting hear from Hurricane Alex. At 70 mph going up overpasses my Jeep didn't downshift a single time. Hats off to 249 drivetrain for a job well done.

 

[MOPWR2U]

Why does Jeep gear these Wranglers so tall from the factory? The Rubi with its 4.10 gears is geared too tall for even the stock 32" tires. Our Rubi with stock gears and tires will turn 2200 RPM at 78 mph, while our Grand Cherokee also with stock gears and tires turns 2700 RPM at 78 mph. Both Jeeps were being driven on the same freeway at the same speed on the same day, my wife was following me in her Grand Cherokee and I called her on her cell phone and asked her how many RPM her Jeep was at.

So why does the RUBICON version of the Wrangler, which is supposed to be the ultimate top of the line OFF ROAD model of Jeep vehicle, have taller (effective) gears than a street only version of a Grand Cherokee? Come on Chrysler, figure it out. The Rubicon should have came with 4.88 gears for the 32" tires it comes with. Or, it shouldn't have such a deep overdrive in the transmission.

 

[The Rhylez]

It's a Rubicon Auto 4.10's pulled 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...

Go 5.13 and be done with it. I have a 2010 jk with km2 37" and with 5.13 it is like stock. Get it done.

 

[tkob1060]

Why does Jeep gear these Wranglers so tall from the factory? The Rubi with its 4.10 gears is geared too tall for even the stock 32" tires. Our Rubi with stock gears and tires will turn 2200 RPM at 78 mph, while our Grand Cherokee also with stock gears and tires turns 2700 RPM at 78 mph. Both Jeeps were being driven on the same freeway at the same speed on the same day, my wife was following me in her Grand Cherokee and I called her on her cell phone and asked her how many RPM her Jeep was at.

So why does the RUBICON version of the Wrangler, which is supposed to be the ultimate top of the line OFF ROAD model of Jeep vehicle, have taller (effective) gears than a street only version of a Grand Cherokee? Come on Chrysler, figure it out. The Rubicon should have came with 4.88 gears for the 32" tires it comes with. Or, it shouldn't have such a deep overdrive in the transmission.

Two different vehicles should NOT be at the same rpm going down the highway. Why would you think that? Different trans ratios, different ENGINES that have make there power in different rpm ranges. #1 reason why any factory vehicle is geared the way it is is because of the having to meet CARB mpg standards.