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Old 04-14-2011, 04:04 PM   #1
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Automatic, 2 door, 35s and 5.38s

For most of my upgrades, Iíve waited until I found I needed them before going out and making them. For example, I took the stock Jeep out on trails in the area until I found that I needed to improve the clearance if I wanted to continue on the trails or try more challenging trails, so I installed a lift. I busted the Evap Can, then relocated it. I found I needed a little taller and wider tire and got 35s. Etc. I was really hoping to keep the 4.10s, butÖ

With the 35s and 4.10s:
1. The first time I drove it I noticed it needed a little more gas to get going. Over time I noticed that it felt more like the gears had a very slight binding/I had to drive up a very small hump when starting - no real binding/bump - just felt that way.
2. On very slight hills and in 2WD or 4High in Drive the transmission canít hold and it rolls back.
3. In 2WD or 4High and going 20mph or less thereís almost no difference between D, 2nd, and 1st (no difference in engine braking, torque).
4. On the freeway it kicks out of OD going over simple/small overpasses.
5. On the freeway going up a steeper hill it kicks down to 2nd to keep up.
6. Going down some steep dirt hills in 4Low had to use brakes.
7. Acceleration from stop not great.
8. Crawling on rocks in 4Low and 1st not much worse than stock.
9. MPG: 32s (14.6K miles): 16.1, 35s (2.2K miles): 15.1

With the 35s and 5.38s:
1. Starting feels great. No extra gas needed.
2. Transmission holds on small hills.
3. Thereís noticeable engine braking and torque in 2nd and a lot in 1st. 4High is nicer on the trail with the engine braking.
4. Doesn't kick out of OD on simple/small overpasses and small hills. Doesn't seem to struggle at all on these types of hills.
5. On larger hills kicks down to 3rd and holds with no struggling going up the hill.
6. On the same hills additional braking is not needed. On most hills no braking is needed at all.
7. Acceleration from a stop is great - no issues.
8. Crawling on rocks in 4Low and 1st better than stock.
9. MPG: Breaking in gears (500 miles): 16.8, My normal driving (8.7K miles): 15.0

The shop I brought it to recommended Yukon 5.13s or 5.38s and after discussing my driving habits and needs we decided on 5.38s. They installed the gears with Front and Rear Master Overall kits and (verbally) they will warrantee the gears for as long as I own the Jeep (assuming I follow their break-in instructions). The upgrade wasnít cheap, but so far it drives as good or better than stock and better than 35s and 4.10s.

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Old 04-14-2011, 04:40 PM   #2
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Thanks for the report. I found the same results when I put 5.38s in my last JK, which was a two door, auto trans, with 35s. I thought that gear ratio was about perfect. I now have 5.38s in a four door JK, auto trans, with 37s, and I think it could stand to be geared lower. 5.67 or 5.86 gears would be perfect for 37s, too bad they don't make them.

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Old 04-14-2011, 08:17 PM   #3
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Originally Posted by MOPWR2U View Post
Thanks for the report. I found the same results when I put 5.38s in my last JK, which was a two door, auto trans, with 35s. I thought that gear ratio was about perfect. I now have 5.38s in a four door JK, auto trans, with 37s, and I think it could stand to be geared lower. 5.67 or 5.86 gears would be perfect for 37s, too bad they don't make them.
Initially I wasnít sure about the 5.38s as that seemed like it was going to be too much, but Iím really glad I got them. Theyíre perfect with my setup and the way I drive it.

I donít think you can go any lower than 5.38s yet.
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Old 07-26-2011, 11:12 AM   #4
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I have read everything I can find on this subject so far. Here's my question ('11 Rubi Unlimited w/ auto and 35's):

Since I'm already going to be adding a complete RIPP long-tube header system and opening up the intake, will 5.13's be better suited than 5.38's? The extra horsepower would tend to make the lower gearing less necessary, wouldn't it? Of course, I can't be certain I will never go with 37's, but I won't be without more drive-train upgrading.

Thoughts?
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Old 07-26-2011, 11:27 AM   #5
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Originally Posted by Stone Mason View Post
I have read everything I can find on this subject so far. Here's my question ('11 Rubi Unlimited w/ auto and 35's):

Since I'm already going to be adding a complete RIPP long-tube header system and opening up the intake, will 5.13's be better suited than 5.38's? The extra horsepower would tend to make the lower gearing less necessary, wouldn't it? Of course, I can't be certain I will never go with 37's, but I won't be without more drive-train upgrading.

Thoughts?
I haven't yet regeared and have a manual transmission anyway, but I've looked at all the charts and read lots of comments, responses, and opinions.

In my opinion . . . I'd still do 5.38s. It's only another 200 RPM or so, and the only practical result is going to be you'll have more power. There's folks on the forum who are very happy with 5.13s on 33s. Headers aren't going to change that.

If you were going to add Ripp's supercharger, I'd expect 5.13s would do. But even then, I'd have to be "talked out of" 5.38s.
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Old 07-26-2011, 02:02 PM   #6
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That is still where my head is too.

My only concern is the Rubi's 4lo gearing. I most often can be found playing in the rocks where the 4:1 gearing is great. But on the trails, its gonna rev a bit high when trying to keep up with others in a group (where 20mph is the same rpm as 80mph highway w/o OD).

I realize the math is linear and it will only be 200rpm above what the 5.13's would be pulling. But that will still be more than the stock 4.10's and 32" tires. I also realize that I can put it in 4hi, but some trails have abrupt obstacles that require 4lo (at least to save the tranny), and stopping to shift the transfer case is a mild pain.

So, in summary, my concerns have nothing to do with on-road drivability -- but off-road.

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