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Unread 12-08-2013, 04:59 PM   #1
kntnrml
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'10 JKU Auto gearing question

Ok, I'm getting ready to re-gear my JKU soon and have narrowed it down to 4.88 and 5.13. I'm currently running nearly new 285's so there is no immediate plans to upsize but it's an option I'd like to be able to consider with the next set of shoes. I know of the strength difference's between them and it is not much of a concern of mine. So, how will it be on 285's with 5.13's, At 70mph towing 1800lbs will it still be downshifting constantly and if so will it be over revving in 3rd gear? How about without the towing load?

And if I go with the 4.88's how will it be if I decide to run 35's later on? I'm assuming it'll be slightly under geared again, especially when towing.

How about the fuel milage differences between the two given that the tire sizes are the same for each gear ratio?

Most of my driving is on relatively flat roads with weekend trips to the mountains of NC if it helps.

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Unread 12-08-2013, 06:05 PM   #2
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if you plan on never going taller than 35s I would go with 4:88. I put 5:13s in mine and ran 35s for a year before I went to 37s. I loved my Jeep all over again on 37s. 5:13s are a perfect match with 37s with a 2010 auto. On 35s, it was never happy. It either over revved in 3rd or it was a slug in 4th and jumped back and forth on hills. I also actually get better mpg with the 37s
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Unread 12-08-2013, 06:15 PM   #3
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I don't know how tall your 285s are but on my 2010 I run 5.38 with 33s and an auto, and I really like it.

I tow 3500# and we have mountains.


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Unread 12-08-2013, 06:18 PM   #4
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wow, I bet your engine really revs at 65-70 mph , No ?
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Unread 12-08-2013, 07:15 PM   #5
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2600ish


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Unread 12-08-2013, 07:18 PM   #6
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I run 5.38s/33.6" Hankooks/auto and get from 15 to 19 mpg-

At 70 mph shes turning a little over 2600 rpm and the best mileage, is 55 mph/2250 rpm/19mpg/0/D

SWEET, plenty power/lightning response/fabulous off-road/towing--up hills/trees/CHV vehicles--

Wubicons happy and so am I !

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Unread 12-08-2013, 08:06 PM   #7
kntnrml
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For those of you with 33's and 5.13's, does it nearly redline when downshifting at 70mph? How much better is it at maintaining speed with OD on?

It's too much work now to try and maintain a hwy speed without cruise control currently which means it's constantly shifting trying to keep pace. If it's still going to be shifting constantly I'd rather it not shift from 2600 to what I'm guessing is about 4k or so?

I'll be making a trip from NC next summer to glacier NP, Yellowstone, Moab, telluride/Ouray and then home via Denver all with 5 on board and an M416 in tow. I really feel like I should run 5.13's...
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Unread 12-08-2013, 08:31 PM   #8
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It will run ok at 4k. Problem is the overdrive is a big step from third.

Really the rpm difference between 5.13 and 5.38 is pretty small.

I like the acceleration off the line with 5.38s, and when not towing it goes into OD and stays there.

When I get to a hill that does drop me out of 4th I go as fast as third gear will let me with 50% throttle as shown on my scan gauge.

These are really marginal as tow rigs, and gearing really helps.

My Jeep, and 2 trailers. imageuploadedbyautoguide1386559902.664049.jpg


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Unread 12-08-2013, 09:04 PM   #9
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You tow the travel trailer with the JK?

Thinking back 1.5yrs, I had a 17' scout center console that weighed about 3k. I could tow it behind the jk when it was stock at 65-70 with od off. It would occasionally downshift to second yet still felt solid even though it was pegging at least 4500. The solution was to simply buy a bigger boat that was out of the jk's ability... Problem solved
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Unread 12-08-2013, 09:23 PM   #10
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imageuploadedbyautoguide1386562913.656871.jpg

Trailer is running a custom 6" lift and 14" wheels to make it handle gravel roads better. Plumbing was only 4" off the ground before the lift.


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Unread 12-09-2013, 07:34 AM   #11
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Quote:
Originally Posted by kntnrml View Post
For those of you with 33's and 5.13's, does it nearly redline when downshifting at 70mph? How much better is it at maintaining speed with OD on?

It's too much work now to try and maintain a hwy speed without cruise control currently which means it's constantly shifting trying to keep pace. If it's still going to be shifting constantly I'd rather it not shift from 2600 to what I'm guessing is about 4k or so?

I'll be making a trip from NC next summer to glacier NP, Yellowstone, Moab, telluride/Ouray and then home via Denver all with 5 on board and an M416 in tow. I really feel like I should run 5.13's...
I drove mine pulling my off road trailer to Corpus Christi 2 years ago when I had it on 35s and 5:13 gears. My trailer was only 800-900 lbs and just my wife and I onboard. I will never do that again. It was a 23 hour drive and the stretch between Houston and Corpus had a head wind that made the Jeep really struggle. It could not stay in OD because of the headwind ( Jeeps have no aerodynamics as you know I'm sure ) and revved at 3600 rpms to maintain 65. The engine overheated and got real sluggish. I finally gave up trying to keep my speed up and just dropped back to 50-55 mph so OD would stay on and the engine wouldn't have to work so hard. I averaged 9mpg With 37s it would've made all the difference, and according to these guys, maybe 33s would've been good too, but 35s were just right to make the transmission hunt for the right gear and never found it.
If you will have 5 aboard and a trailer, I would load up the way you will for your trip and drive to Boone for a day trip to see if you can stand it or not. You will definitely need to get the gearing/ tire size right before you drive that far with that much weight.
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Unread 12-09-2013, 08:04 AM   #12
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Quote:
Originally Posted by Wubicon View Post
It will run ok at 4k. Problem is the overdrive is a big step from third.

Really the rpm difference between 5.13 and 5.38 is pretty small.

I like the acceleration off the line with 5.38s, and when not towing it goes into OD and stays there.

When I get to a hill that does drop me out of 4th I go as fast as third gear will let me with 50% throttle as shown on my scan gauge.

These are really marginal as tow rigs, and gearing really helps.

My Jeep, and 2 trailers. Attachment 904530


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I agree here. Not much difference in rpms. For one its a v6 motor v style motors are made to run higher rpms than the straight 6s were. Id feel safe even running 3200 rpm on the freeway. Id do 5.13 on a d30 all day and 5.38 all day on a d44. Hell even on my 5.13s towing a trailer i run over 4k rpm up some steep hills. No problems yet.
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Unread 12-09-2013, 08:08 AM   #13
kntnrml
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Quote:
Originally Posted by Joevacs View Post
If you will have 5 aboard and a trailer, I would load up the way you will for your trip and drive to Boone for a day trip to see if you can stand it or not. You will definitely need to get the gearing/ tire size right before you drive that far with that much weight.
I make the trip to Boone several times a year both loaded and unloaded. It's damn near unbearable either way.. Seems crazy that a matter of 120rpm can make such a difference with gear searching at 70mph.

The towing I do makes me think the 5.13 would be better with the 33's but elsewhere I've read 4.88 is the way to go with 33's. I've got about 30 days to make a decision though so hopefully I'll hear from others with a similar setup as mine

SB
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Unread 12-09-2013, 08:14 AM   #14
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Sorry about the headwinds Jv, but I've been towing my off-road trailer (1000 lbs) for over two years with my 5.38s and


Quote:
Originally Posted by Joevacs View Post
I drove mine pulling my off road trailer to Corpus Christi 2 years ago when I had it on 35s and 5:13 gears. My trailer was only 800-900 lbs and just my wife and I onboard. I will never do that again. It was a 23 hour drive and the stretch between Houston and Corpus had a head wind that made the Jeep really struggle. It could not stay in OD because of the headwind ( Jeeps have no aerodynamics as you know I'm sure ) and revved at 3600 rpms to maintain 65. The engine overheated and got real sluggish. I finally gave up trying to keep my speed up and just dropped back to 50-55 mph so OD would stay on and the engine wouldn't have to work so hard. I averaged 9mpg With 37s it would've made all the difference, and according to these guys, maybe 33s would've been good too, but 35s were just right to make the transmission hunt for the right gear and never found it.
If you will have 5 aboard and a trailer, I would load up the way you will for your trip and drive to Boone for a day trip to see if you can stand it or not. You will definitely need to get the gearing/ tire size right before you drive that far with that much weight.
Even though I'm able to maintain 65 mph in O/D, I will alternate(in/out) it every 25 miles because the auto temp climbs slowly using the O/D while towing-

I will drop outta O/D when she(auto) gets to 200F, not to high, but I don't wanna stretch my advantage/w gears-

As I've said, my JKUR/5.38s/auto will maintain 14.9 mpg while towing on freeway-at 65mph

It's a jungle out there

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