6 speed gear decision input needed

[CorvZ061]

I’m about to get my wife’s Jeep regeared. Hers is also a 6 speed but a sport. Her Sport has my original Rubi axles under it with the stock 4.10’s. We recently did a small lift and 35x12.5-17 tires on my original Rubi wheels. Hers will never go bigger than 35’s and the 4.10’s just don’t cut it on the highway. We recently went to OKC from Amarillo and was in 5th pretty much the entire just to be able to go the speed limit of 75. 6th couldn’t maintain that speed even on flat road.

Those that have 35’s on a 6 speed, 4.88 or 5.13? I’m leaning towards 4.88 for the bigger pinion, but that’s not a huge deal on hers it’ll see some dirt roads and light trail use

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[roaniecowpony]

3.6?

That's a lot of gear at 75mph with 35s. You are kinda in a rpm range where you are starting to have plenty of power with 4.88s, but it's going to wind pretty tight. 5.13s may be a bit much for 35s.

 

[CorvZ061]

3.6?

That's a lot of gear at 75mph with 35s. You are kinda in a rpm range where you are starting to have plenty of power with 4.88s, but it's going to wind pretty tight. 5.13s may be a bit much for 35s.

Pretty sure 3.6 is the only engine option with a manual trans.

 

[roaniecowpony]

Pretty sure 3.6 is the only engine option with a manual trans.

Well, my thought is 4.56 would be about right. But, changing from 4.10 to 4.56 would be a subtle change. BTW, I'm on 37s with 4.88s with an auto.

 

[AVGeek99]

Strange that you have that level of issues running 35s with 4.10s. I put 40k miles with 35s on my otherwise stock 21 JLUR 6MT. I wouldn't say it was ideal in hill country, but it was perfectly acceptable 95% of the time. I rarely had no issues maintaining highway speeds in 6th gear on the interstate, except when I was in the mountains. The one thing that would give it trouble in flatter territory was a strong headwind, then it would struggle.

I lifted and regeard to 4.88s at 40k miles. Drove it for a few months before moving to 37s. I honestly can't say I liked 4.88s with 35s, too short, especially 1st. I was much happier when I finally put the 37s on. If I was never going to go bigger than 35s I wouldn't go higher than 4.56s. But that's a lot of money for only one gear step.

 

[Oletimer]

I had 4,88s with 35s on my last one. I think it was perfect.

 

[roaniecowpony]

Strange that you have that level of issues running 35s with 4.10s. I put 40k miles with 35s on my otherwise stock 21 JLUR 6MT. I wouldn't say it was ideal in hill country, but it was perfectly acceptable 95% of the time. I rarely had no issues maintaining highway speeds in 6th gear on the interstate, except when I was in the mountains. The one thing that would give it trouble in flatter territory was a strong headwind, then it would struggle.

I lifted and regeard to 4.88s at 40k miles. Drove it for a few months before moving to 37s. I honestly can't say I liked 4.88s with 35s, too short, especially 1st. I was much happier when I finally put the 37s on. If I was never going to go bigger than 35s I wouldn't go higher than 4.56s. But that's a lot of money for only one gear step.

I wonder if he got the torque management software for the clutch issue and it's restricting.

 

[Oletimer]

I wonder if he got the torque management software for the clutch issue and it's restricting.

mine i felt was the weakest thing I ever owned. I put 40 miles on it and decided I couldn't live with it. 4.10s with 35s
and I had already owned several jeeps.

 

[eastern]

Never met anyone regretting lower ratio, posts about wishing having lower ratio are numerous.

 

[CorvZ061]

Strange that you have that level of issues running 35s with 4.10s. I put 40k miles with 35s on my otherwise stock 21 JLUR 6MT. I wouldn't say it was ideal in hill country, but it was perfectly acceptable 95% of the time. I rarely had no issues maintaining highway speeds in 6th gear on the interstate, except when I was in the mountains. The one thing that would give it trouble in flatter territory was a strong headwind, then it would struggle.

I lifted and regeard to 4.88s at 40k miles. Drove it for a few months before moving to 37s. I honestly can't say I liked 4.88s with 35s, too short, especially 1st. I was much happier when I finally put the 37s on. If I was never going to go bigger than 35s I wouldn't go higher than 4.56s. But that's a lot of money for only one gear step.

Wind is always an issue here and what contributed to the miserable drive.

 

[Joe's_Roxy]

I regeared my 18 Rubicon to 4.88 gears after putting on my 35s and it handles all speeds very well. Brings the RPMs up to a very manageable range in 6th gear (if I remember right, I see about 2500 RPMs at 70mph).

My initial thought for you would be 4.56 gears unless you are wheeling with her Jeep but I have been very pleased with my 4.88 gears...both on and off-road.

If you are planning on wheeling her Jeep, with the Sport Transfer case, you will need to go to a higher gear ratio...such as 5.13s or more...to get easier driveability off-road over obstacles...or put in the Atlas Transfer Case you know you want to put in anyway.

 

[JT1]

I’m about to get my wife’s Jeep regeared. Hers is also a 6 speed but a sport. Her Sport has my original Rubi axles under it with the stock 4.10’s. We recently did a small lift and 35x12.5-17 tires on my original Rubi wheels. Hers will never go bigger than 35’s and the 4.10’s just don’t cut it on the highway. We recently went to OKC from Amarillo and was in 5th pretty much the entire just to be able to go the speed limit of 75. 6th couldn’t maintain that speed even on flat road.

Those that have 35’s on a 6 speed, 4.88 or 5.13? I’m leaning towards 4.88 for the bigger pinion, but that’s not a huge deal on hers it’ll see some dirt roads and light trail use

I wouldn't do 5.13s with 35's. I'm on 37s and 5.13s and when we head down the turnpike your direction, I'm in 6th at ~2800 doing the speed limit. The second stage of the oil pump kicks in at 3000 rpm and mileage takes an even bigger hit. 4.56s in 5th is the same effective ratio as 5.13s in 6th, but you still have another gear.

Use the calculator here: https://www.grimmjeeper.com/gears.html and see what RPMs you had to turn to maintain speed on that drive and find a ratio (I'm guessing 4.56 or 4.88) that puts you at the speed you normally drive on the highways and near the RPM you needed to pull the hills. See which one keeps you under 3000 rpm at the cruising speed you normally run.

 

[AlgUSF]

I'm on 35's with the 6MT and geared to 4.56, works great for me. I have the Dana 30/44 combo in the Sport. 5th and 6th gear are both overdriven and so close in ratio, I wouldn't worry about hanging out in 5th.

 

[Traveller128]

I’m about to get my wife’s Jeep regeared. Hers is also a 6 speed but a sport. Her Sport has my original Rubi axles under it with the stock 4.10’s. We recently did a small lift and 35x12.5-17 tires on my original Rubi wheels. Hers will never go bigger than 35’s and the 4.10’s just don’t cut it on the highway. We recently went to OKC from Amarillo and was in 5th pretty much the entire just to be able to go the speed limit of 75. 6th couldn’t maintain that speed even on flat road.

Those that have 35’s on a 6 speed, 4.88 or 5.13? I’m leaning towards 4.88 for the bigger pinion, but that’s not a huge deal on hers it’ll see some dirt roads and light trail use

Tune it. Get hold of Brad on the tuning thread. You'll have plenty of power to drive 75 unless you live at high altitude. I was getting ready to bump our 4.10's to 4.56 going to 34" tires and now I'm not feeling the need. I think the 4.56 set would be ideal, but don't feel the need to spend the money. Plenty of power to drive 75-80.

 

[JeepinPete]

I ran 4.10' and 37" KO2 for 40k miles. It worked and really didn't see any difference in mpg running 4th, 5th or 6th on the highway (70-85mph). Regeared to 5.38's last fall, still running 37's. Probably a bit much for most folks but I have started towing with mine, wanted to remove as much load from the clutch as possible. Around 80mph I hit 3k rpm. I'll run it up to 85mph, but I do pick up a slight drivetrain vibration above 80mph now, no doubt the driveshaft since its singing at 4000 rpm. Lost about 1mpg in total.

In your case I would just run it in 5th on the highway. But 4.88 would be plenty deep if you are going to change gears.

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