Performance impact of 35s with Rubicon 4.10 gears

turbosix

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I apologize for not being more specific in my question. 6th gear is purely intended to be overdrive. I would never expect it to accelerate quickly in 6th regardless of tire size. I would also expect 6th to be pretty useless on hills that most would consider steep.

I guess I could have explicitly asked "will it hold speed (70-85mph) with light headwinds (10-15mph or less) on normal interstate (non "steep grade") hills.

I do like the gear ratio tools and dyno charts, but I'm not enough of a gear head to know when it will have enough power to hold speed at certain RPM levels. According to the Gear Ratio calc tools 35" tires with 4.10 gears at 2500rpm will result in a speed of 86mph. Ok, but does it have enough power to hold 2500rpm with 35" tires?

My wheels and tires will be XD137 FMJs 20x9 ET0 with Cooper Discoverer STT Pros 35x12.5 R20.

The only thing I have to compare too right now is my JK and my memory of what it was like when it had the stock 3.21s. It would barely do 80mph. Granted 3.21s is a LOT higher than 4.10s, the JK 6spd is geared lower than the JL, but not that much lower. Knowing this I expect the JLUR with 35s won't be as bad as my JKU Sahara on stock 3.21s, but is 6th gear on a JLUR useable as a pure overdrive in the real world, or will it not hold speed in most cases?
My man. Much better. 🤙
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AVGeek99

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I played around with one of the gearing calculators to try to compare JL gearing to JK gearing that I am familiar with. As far as I can tell the 3.6L Pentastar V6 has changed very little if at all from 2012 to 2021, having the same HP and torque. The transmissions however are geared differently. 1st is lower on the JL, 2nd is almost identical and then 3rd through 6th are all higher on the JL.

In the graphs below, the solid lines are for the JL, the dotted lines are the JK. All graphs are based on 35" tires on both the JL and JK. The main focus of my comparison is with 6th gear, trying to figure out how useable it will be with 4.1 gears and 35" tires.

My JK had stock 3.21 gears when I got it. It got me around fine but it was a pig, 5th gear was fine for highway driving, 6th only held highway speeds when there was zero headwind and incline. The slightest incline would cause it to lose speed quickly, even with it floored, which killed mileage. I very quickly learned not to bother with 6th gear at all. 1st gear in the JK was also a problem starting out on steep hills. Once or twice I had to drop it to 4Low to get it to a lesser incline.

Comparing the stock JLUR to the stock JKU Sahara, I can see that the JL should be a decent amount better than the JK. 6th and the JL will be a little higher than 5th on the JK. And with 1st being so low on the JL, it shouldn't ever be an issue.

1630816601954.png


The most even comparison between the JL and JK is a stock JL and a JK with 3.73s. With this setup 6th gear is virtually identical.

1630818584340.png


This next graph shows how the stock JL compares to my current JK set up with 4.56s. 1st gear is virtually identical with the JL actually being a smidge lower. But on the higher end 6th gear in the JK is almost right in the middle between 4th and 5th in the JL.

1630818769943.png


Moving to 4.56s on the JL is interesting. It's almost like it has one more overdrive gear than the JK. 6th gear in the JK holds highway speeds (75 to 85) very well. I wonder how well the JL would be able to hold those speeds in 6th.
1630819343052.png


4.88s on the JL are the most equivalent to 4.56s on the JK. 6th is still a little higher on the JL. If I end up regearing I'll probably go with 4.88s.

1630819542088.png
 

Krondor

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With 35s, sounds like 4.88s on manuals is the sweet spot for people who highway and trail their Jeeps. Correct me if I am wrong but you seem to be very pleased with your 4.88s/35tires/MT, Yeah?

I would also go as far to say that 4.56s is gearing Jeep should have in the standard MT Rubicon and 4.10s in all the lower trim MTs.
I agree. The standard Rubicon should have come with 4.56s from the factory with this 6 speed's gearing.

The reverse gear is much too high as you can't maneuver a trailer up a good hill without burning clutch or pushing the trailer too damn fast. I'll work around it. But with 4.56 and 33's you would get the responsiveness and crawling I get now with 4.88's and 35's. Yes, I even get a bit better mileage.

Really love the wheeling now. I push up trails in 2wd for as much as I can, and the first gear gets me a lot further these days. lol
 

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Newbie question...
What are the pros/cons for a 6-speed JLUR running 35's with 4.10 vs. the same with 4.88?
Thanks!
 

Krondor

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Newbie question...
What are the pros/cons for a 6-speed JLUR running 35's with 4.10 vs. the same with 4.88?
Thanks!
I thought the 2 door with 6 speed did just fine on 4.10's to be honest.

Then I got re-geared for 4.88's. Absolutely worth it. The Jeep feels much more peppy and even gets better mileage. Crawling in 2nd and 3rd is great, while first is beast mode. Reverse is still too damn high.

Would I have liked 5.13's better? Yeah, but then highway driving would start to suffer. IMHO

I intend to do some driving to other states, so I'm very happy with 4.88's. If you want to save money, the 4.10's will do the job fine. If you wheel, you will love the extra grunt. If you ever intend to go to 37's, jump to the 5.13's. :beer:
 
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AVGeek99

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Newbie question...
What are the pros/cons for a 6-speed JLUR running 35's with 4.10 vs. the same with 4.88?
Thanks!
Stock JLUR comes with 33s and 4.10 diffs. Increasing the tire size reduces power/torque to the tires. Increasing tires to 35s on the 4.10s is equivalent to running the stock 33s with a 3.83 gear ratio. The overall impact to how the vehicle operates ultimately is dependent on the gearing in the transmission and how much power/torque headroom existed with the stock 33/4.10 configuration.

Where you will most feel the impact of moving to 35s with the stock 4.10s is in 6th gear. 6th gear in the JLs 6spd is geared very tall (i.e. high), so increasing tire size will reduce the ability for the vehicle to maintain speed going down the highway. Depending on how hilly/windy it is where you live 6th gear may or may not be very useful with 35s and 4.10s.

When I first got my JK it had 35s on stock 3.21 diffs. 6th gear was useless, so I never used it at all. Ultimately I upgraded to 4.56s and it was a completely different vehicle and all my power problems were solved.

The JL manual transmission is geared taller than the JK though. 6th gear in the JK is between 5th and 6th in the JL, but closer to 5th. With the taller high end gears in the JL 6spd 4.88s will give you the most useable 6th gear in a wider range of situations on road.

The benefits of 4.88 dffs on a JLUR off road really depend on what type of wheeling you do. With the 4:1 4Lo transfer case ratio, first gear gives you a crawl ratio of 100:1, this is useful in hard core rock crawling, but in most trail situations you'll be somewhere between 2nd and 4th.

My 2021 6spd JLUR will be delivered within a week or two. I'm having the dealer put on 35s from the get go. Based on my experience with my JK and knowing the JL is geared taller I'm kind of thinking 6th gear won't get much if any use, though I hope I'm wrong. I will likely end up regearing and when I do it will likely be to 4.88s.
 
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