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Unread 08-27-2010, 10:01 AM   #1
orange_rubi
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let's go over this one more time ... recommend gears for my 35" tires

I know this question has been answered a bunch of times but I really want some direct answers.

I am moving from AZ (no freeway driving - all city) to CA (will be all freeway driving) and using the jeep as a DD. I had originally thought about getting another vehicle and using that as my DD (but I plan on proposing to the GF in the near future and would rather use that money to buy her a nicer ring).

So I need gears - I've been putting it off because around town the 4.10s have been ok and wheeling they've been bearable.

For the best FREEWAY performance do I go with 4.88's or 5.13's - see profile or signature below for specs (and it's the 4 speed auto)? Is the weight/length of the unlimited worth going to 5.13's? I'm going to be traveling about 60 miles a day on the freeway and I know that whichever I get will outperform the 4.10's wheeling. It's all about highway speed and fuel economy (all 14 mpg or so of it )

I am confused 'cause the chart says my best fuel economy is coming from the 4.10s when on 35's (part of the reason I stuck with them for now) - I find this hard to believe that this holds true when going 65-70 down the freeway.



Please help - if you're going to flame me about doing a search please don't respond. I have read plenty just want the reassurance because I can only afford one shot at this and I'll be driving my jeep from AZ to start the new job in CA in two weeks. Thanks!

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Unread 08-27-2010, 10:11 AM   #2
NotURMailman
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I have 4.88s with my 35s and 42rle, I think they are fine. But, I have never driven one with 5.13s. Your performance should be closer to stock with 5.13s, but you may see slightly better fuel ecomony with 4.88s. If I had a Dana 44 front axle I certainly would have gone with 5.13s, but I have other vehicles and was more concerned with trail performance. I have driven my Jeep from upstate NC to Charleston SC with the 35s and 4.88s and it did fine. My mileage is around 13, where stock was 17. Then again, I never gave fuel ecomony a single though while I was building it and don't really care now...
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Unread 08-27-2010, 10:29 AM   #3
Jerry Bransford
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Quote:
Originally Posted by orange_rubi View Post
I know this question has been answered a bunch of times but I really want some direct answers.

I am moving from AZ (no freeway driving - all city) to CA (will be all freeway driving) and using the jeep as a DD. I had originally thought about getting another vehicle and using that as my DD (but I plan on proposing to the GF in the near future and would rather use that money to buy her a nicer ring).

So I need gears - I've been putting it off because around town the 4.10s have been ok and wheeling they've been bearable.

For the best FREEWAY performance do I go with 4.88's or 5.13's - see profile or signature below for specs (and it's the 4 speed auto)? Is the weight/length of the unlimited worth going to 5.13's? I'm going to be traveling about 60 miles a day on the freeway and I know that whichever I get will outperform the 4.10's wheeling. It's all about highway speed and fuel economy (all 14 mpg or so of it )

I am confused 'cause the chart says my best fuel economy is coming from the 4.10s when on 35's (part of the reason I stuck with them for now) - I find this hard to believe that this holds true when going 65-70 down the freeway.



Please help - if you're going to flame me about doing a search please don't respond. I have read plenty just want the reassurance because I can only afford one shot at this and I'll be driving my jeep from AZ to start the new job in CA in two weeks. Thanks!
Such charts are very misleading in that they only give the RPM for when your transmission is in its 1.0:1 ratio. That means if you have a 5-speed manual transmission, those rpms represent what you would see while driving around in 4th gear.

With your 4-speed automatic transmission, those RPMs would only be seen when the overdrive is turned off.

With your 42RLE 4-speed automatic, not even 4.88 is low enough, 5.38 is actually the right choice for 35" tires. I am running 35" tires with the same transmission you have and have 4.88 gears that the previous owner installed. My engine lugs and I keep having to turn the overdrive off. If I had the $$$, I would regear my Rubicon to 5.38.

Since you are driving an Unlimited and not a Rubicon, your front axle is a Dana 30 and the lowest ratio that can be installed into it is 4.88. Don't even THINK about anything but 4.88 with your particular transmission. You would hate 4.10 or 4.56.
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Unread 08-27-2010, 10:36 AM   #4
wpparker
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I know someone with a 05 rubi. running 35's and 5.13 gears and loves them. His wife drives it 80 miles to the next town at 70mph with no problem. It does not tach any more rpm's than my 3-speed auto with 33's and 4.56.
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Unread 08-27-2010, 10:59 AM   #5
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Originally Posted by NotURMailman View Post
IYour performance should be closer to stock with 5.13s, but you may see slightly better fuel ecomony with 4.88s. If I had a Dana 44 front axle I certainly would have gone with 5.13s ...
NotURMailman: Good to note - so that's one vote for 5.13's. I would like to get it closer to stock - remember driving it on the freeway stock and was surprised how "smooth" it ran.

Quote:
Originally Posted by Jerry Bransford View Post
Such charts are very misleading in that they only give the RPM for when your transmission is in its 1.0:1 ratio. That means if you have a 5-speed manual transmission, those rpms represent what you would see while driving around in 4th gear.

With your 4-speed automatic transmission, those RPMs would only be seen when the overdrive is turned off.

With your 42RLE 4-speed automatic, not even 4.88 is low enough, 5.38 is actually the right choice for 35" tires. I am running 35" tires with the same transmission you have and have 4.88 gears that the previous owner installed. My engine lugs and I keep having to turn the overdrive off. If I had the $$$, I would regear my Rubicon to 5.38.

Since you are driving an Unlimited and not a Rubicon, your front axle is a Dana 30 and the lowest ratio that can be installed into it is 4.88. Don't even THINK about anything but 4.88 with your particular transmission. You would hate 4.10 or 4.56.
Jerry: I actually do have a Rubicon Unlimited so I have dana 44's f/r - so you are saying go higher than 4.88's - 5.13's or 5.38's (what about 5.29's?). Always value your assistance/insight! Thank you!


Quote:
Originally Posted by wpparker View Post
I know someone with a 05 rubi. running 35's and 5.13 gears and loves them. His wife drives it 80 miles to the next town at 70mph with no problem. It does not tach any more rpm's than my 3-speed auto with 33's and 4.56.
wpparker: Thanks. So far it's sounding like I might be happier with 5.13's as opposed to the 4.88's - glad I asked!
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'81 cj-5 - being restored w/ dad - keeping the 151
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Unread 08-27-2010, 11:10 AM   #6
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Quote:
Originally Posted by orange_rubi View Post
Jerry: I actually do have a Rubicon Unlimited so I have dana 44's f/r - so you are saying go higher than 4.88's - 5.13's or 5.38's ?
Acutally he's saying go LOWER!
Sorry!

RJ
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Unread 08-27-2010, 11:10 AM   #7
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Quote:
Originally Posted by orange_rubi View Post
Jerry: I actually do have a Rubicon Unlimited so I have dana 44's f/r - so you are saying go higher than 4.88's - 5.13's or 5.38's (what about 5.29's?). Always value your assistance/insight! Thank you!
Yep sorry about that, I should have noticed that simply by your name.

Technically, 4.88 is a lower ratio than 4.10 is. 5.13 would be better but I would go to 5.38. You are fortunate that you have a Rubicon and can go with a lower ratio than 4.88 which just isn't low enough of a ratio for 35" tires when you have the 4-speed automatic.
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Unread 08-27-2010, 11:22 AM   #8
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This is also a great tool to help you decide: http://www.grimmjeeper.com/gears.html

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Unread 08-27-2010, 11:24 AM   #9
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As has already been mentioned, that chart is giving you the 1:1 RPM's (your 3rd gear). Multiply the RPM's by .69 (your OD ratio) and you'll get RPM's in OD. Due to your overly tall OD, you need to gear lower than the rest of us. Go 5.13. I didn't think Rubi axles could be geared to 5.38 with the stock carriers....but I could be wrong. If they can, go that route. The thing will feel like a rocket with 5.38's and you'll never be lugging the engine.
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Unread 08-27-2010, 11:43 AM   #10
orange_rubi
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Originally Posted by RowJ View Post
Acutally he's saying go LOWER!
Sorry!

RJ
my bad - I didn't proofread

ok so 5.13's as HIGH as I want to go but if I can go LOWER - I will ... read it twice before I posted to make sure I got it right

Thanks guys!
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'81 cj-5 - being restored w/ dad - keeping the 151
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Unread 08-27-2010, 11:45 AM   #11
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I'm running 35s with the 4.88s (42rle) and I can tell you I wish I had more on the trails. On some of the longer, smooth hill climbs at our local park it just seems to run out of gear. Highway-wise it gives out around 70 MPH. 4.88s are easier to live with than the sock 3.73s, but I should have saved money for a new front axle and gone bigger.
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Unread 08-27-2010, 12:53 PM   #12
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Correct 5.13's are the lowest you can go without changing the carrier. 5.38s would be the best for the 35s in this case. 5.13s are starting to pop up on 4 speed auto Jeeps with 33s alot more.
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Unread 08-27-2010, 02:14 PM   #13
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With the Rubi, the lowest you can go (highest numerically) and keep your factory air lockers is 5.13. Since you have the Rubi, do yourself a favor and skip the 4.88's; go right up to 5.13's.

As soon as I can afford a new front axle, I'll be going to 5.38's with my 35's. The current 4.88's, like Jerry said, are just too high with the 42RLE.
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Unread 08-27-2010, 02:55 PM   #14
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I wasn't aware the Rubi's locker case couldn't take 5.38 gearing and that 5.13 was as low as can be installed onto it. I wonder if that could possibly be accomplished with a "fat" 5.38 R&P gear set.
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Unread 08-31-2010, 10:42 AM   #15
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all great info and just to update all those who posted (and i thank you for your responses) ... i still have not decided. :

so 5.13s are the general consensus from jf (although I did a search back and noticed that not more than 2 years ago many people were saying that 4.88s were the way to go ... so that has me confused). i've got another group of people that live in la telling me that 4.88s will be better if it's my dd (especially since now mileage will be split more along the lines of 90% dd and 10% offroad as opposed to the 70-30 it is now). i have a guy at one shop saying 4.88s because the pinion head on the 5.13s is smaller and actually making less contact with the ring thereby increasing my chance of chipping a tooth. my main guy who i go to for all advice and helped install my la kit (and a well respected individual of some hard-core offroad vehicles) is saying 5.13s but that's because he's looking at more on the trail aspect. it's currently 6-to-one and half-a-dozen-to-the-other.

i'm honestly not indecisive - i just do a lot of research and gather as many opinions as possible before i jump into anything. i'll keep you all posted and in the meantime if anyone has anything else to add please feel free.
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'81 cj-5 - being restored w/ dad - keeping the 151
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