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Old 01-03-2018, 12:29 PM
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Exclamation Newbie Question: Gears?!?

Hello! I have a 2015 Unlimited Rubicon HR I just purchased. It's not my first Jeep, but my first lifted Jeep since my cj-7. I'm not sure of the size of lift, which I would like to figure out. It is a 6 speed manual running with lt305 70 18 Mickey Thompson ATZ shoes. It still has the stock 4.10 gears. It gets approximately 11.3 mpg before I found 87 octane no ethanol gas which bumped it to 12.6. The Jeep wanders a lot on the interstate. I'm having an alignment done next week to verify the toe-in. I dropped the air pressure from 37 to 29psi. Smoother ride, no difference in wander. Which gears should I install, 4.88 or 5.13? This is my normal commuter vehicle now with 24k on the clock and 76 miles round trip for work.

Thank you for your assistance and knowledge.

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Old 01-03-2018, 02:26 PM   #2
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If you take some picture of your suspension maybe we can help you out on the lift.

As far as gears go 4.88 is what most of us with manuals have. Works well with 35's with the option to go to 37's in the future. 5.13 is mostly considered a 37 and up gear unless you live at altitude.

FYI: Running 4.10's and 35's I avereage 15.4 mpg, running 4.88's now I'm at 15.3.


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Old 01-03-2018, 04:15 PM   #3
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As far as fuel mileage goes, has it been reprogrammed for the bigger tire size? I run the exact tire combo as you and I get 15 to 16 mpg(but drive it tenderly on the hwy).

As I am sure you know, gears have nothing to do with it wandering. Need to check all the torques on your drivetrain components as well.

There are a lot of people that run 4.10's with your size tire and have no issues.
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Old 01-04-2018, 09:09 AM
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As far as I know, I think it has been. the speedometer is correct when checked against a GPS unit. I never shift above 2k rpms, most of the time between 1.5-1.75k. Been driving the interstate at about 62 verses 70. Have a shop going to do an alignment on it next Thursday. Aired the tires down to 28psi and seems to be a much smoother ride. I did figure out the lift kit that is installed. It's a MetalCloak Fox edition 3.5" lift.
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Old 01-04-2018, 11:39 AM   #5
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If you're going to crawl at all, install 5.13 and go with 37s and make sure you change out your drive shafts. You will be much happier with the results. JEEP Just empty ever pocket. HAHAHA

Question for other out there.

Anyone using the Banks autowind and changed their low Xfer case setting?
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Old 01-05-2018, 09:46 AM   #6
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For gears, I agree I would look at the 4.88's. I would also do a master install kit. This will get you new bearings. Pulling and re-pressing old bearings is not the best idea. Best to just replace them when your in there. This kit is what we usually run:
Also, keep in mind that while you are doing the gears, it is the PERFECT time to upgrade to a locker of some sort as well as swap in new axle shafts. We can help there as well.

For the wandering, the alignment should help. Make sure the shop sets the castor where it needs to be. I would start there. If it still has issues, let us know and we can look at other ideas.

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Old 01-05-2018, 03:44 PM   #7
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I would also suggest considering 4.56's.

I have them in my 17 JKU with 35's and the RPM's are about 2500 on the fwy. That will help with fuel consumption on the highway.

If you do any towing or crawling, then you should consider the 4.88's.

But if fuel economy is your primary concern, take a look at 4.56's.

If course, If fuel economy is a major concern, you bought the wrong vehicle. LOL.
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Old 01-05-2018, 04:10 PM   #8

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Originally Posted by Fixmy59bug View Post
I would also suggest considering 4.56's.

I have them in my 17 JKU with 35's and the RPM's are about 2500 on the fwy. That will help with fuel consumption on the highway.

If you do any towing or crawling, then you should consider the 4.88's.

But if fuel economy is your primary concern, take a look at 4.56's.

If course, If fuel economy is a major concern, you bought the wrong vehicle. LOL.
While I agree that 4.56's are an option with 35's, I don't think fuel mileage will be much better with them. How you drive will make more difference than whether you have 4.56 gears or 4.88 gears. And the increase in perceived power will be bigger than the possible increase in fuel mileage.
Some people don't like the extra couple hundred rpms you will pull at 70 mph between 4.56 and 4.88 gears. And if that is the way someone feels, that is fine. My sister in-law went with 4.56 gears for her 2016 JK / automatic. I think she already kinda wishes she had gone with 4.88 gears.
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Old 01-05-2018, 05:26 PM   #9
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There's been multiple threads on gears and gear changes for the JK both the 3.6L and the 3.8L engines. Our '15 JKUR Auto came with 3.73s. I outfitted the Jeep with 315 A/Ts and, through lots of experience, and inquiries, I decided to go with 4.56s. The throttle response with the 4.56s is considerably better and is less needed for any given driving scenario, i.e. stop light and stop sign take-offs, hills, grades, freeways and more. My freeway rpms went up right at about 350 rpm from the 3.73s.

If you don't plan on moving up to 37s which, adds all kinds of other serious considerations i.e. bigger brakes, axles, possible adjustments to your present lift, I'd hang with the 4.56s. You'll get great all around driving improvement. As for "crawling", don't get caught up with the ultra hard core folks. I've ran all kinds of jeeps with all kinds of lifts, tire sizes and gear sets. 4.56s with 35s and below, work flawlessly. The amount of throttle needed in first gear-low range for crawling and, the speed at which your moving, it right close to idle.

Only you know what you'll attempt in "hard core" crawling so, running your 305s and 4.56s will for the most part, get you through anything within reason of your Jeeps presently outfitted capability.

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