Manual Transmission - Issues shifting into 1st & Reverse (Compilation thread)

[Toycrusher]

So adding to this thread:
I had an interesting convo with my mechanic. He called several companies and talked to them about the whole clutch situation. In his opinion of working with and on performance and off road vehicles- McLeod seems to be the ticket, based on the design and their knowledge of the subject.

He also said told me some of the stuff McLeod has been looking in to- basically, they seem to think there is a correlation between People with larger tires running sub-optimal axle gearing and a much higher failure rate, logic being it takes more slipping of the clutch to get/ keep things moving.

Food for thought, if nothing else.

Larger tires and taller gearing absolutely results in additional clutch disc wear, as well as heat damage to the flywheel and pressure plate. However, clutch wear would not affect shift quality. If anything, it means additional clearance when depressing the clutch pedal which would only lessen the difficulty shifting. Most of the shift issues reported on these forums are related to the inability of the synchros to match input and output shaft speeds for gear engagement

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[GabeBoyTheGreat]

Well, I've got 37" tires with 4.10's. The guy at McLeod basically said "37's, he should be on 5.13's!" to which my Mechanic replied "Yes, but It's a daily driver he drives on the highway. 4.88's would probably be top". I believe the guy from McLeod said most of the users they had worked with that experienced a failure were 4.56 or lower. He made the point that if Jeep is doing the XR package with 4.88's and 35's w/ the manual transmission and V6 (per the builder on the website) and that is considered "optimal" by the OEM, then it would end up having to be a numerically higher (lower) gear ratio for 37's.

I don't know the mathematics and engineering behind it, but the logic checks out i guess.

I agree that 4.88s/35s is the optimal combo with the 6MT, but not because the OEM considers it optimal. Remember, this is the same OEM that put this garbage clutch in the first place and stills offer 3.45s as the base gearing for the 6MT.

5.13s on 37s would be about equivalent to 4.88s on 35s. The simple math is, the % increase in tire diameter should be equal to the % increase in the numerical gear ratio to give the same performance.

IMO, there are 3 gear ratios for a given tire size that are considered "reasonable." You have the optimal gear ratio, one step numerically lower for economy, and one step numerically higher for performance. Anything outside of those 3 I would consider extreme.

IMO, 5.13s would be the optimal ratio for 37s, 4.88s would be the economy, and 5.38s would be the performance. Normally, I like to go for the economy ratio for a DD, but considering the high 6th gear, wide spacing between the lower gears, and lightweight flywheel, I wouldn't settle for the economy gears on the JL. For me, I plan to go with the optimal ratio.

 

[Toycrusher]

I agree that 4.88s/35s is the optimal combo with the 6MT, but not because the OEM considers it optimal. Remember, this is the same OEM that put this garbage clutch in the first place and stills offer 3.45s as the base gearing for the 6MT.

5.13s on 37s would be about equivalent to 4.88s on 35s. The simple math is, the % increase in tire diameter should be equal to the % increase in the numerical gear ratio to give the same performance.

IMO, there are 3 gear ratios for a given tire size that are considered "reasonable." You have the optimal gear ratio, one step numerically lower for economy, and one step numerically higher for performance. Anything outside of those 3 I would consider extreme.

IMO, 5.13s would be the optimal ratio for 37s, 4.88s would be the economy, and 5.38s would be the performance. Normally, I like to go for the economy ratio for a DD, but considering the high 6th gear, wide spacing between the lower gears, and lightweight flywheel, I wouldn't settle for the economy gears on the JL. For me, I plan to go with the optimal ratio.

For thick humid S. Texas air, you would need 5.38s to bring 6th gear to a usable point. I've meant to get around to it forever but I've honestly been driving for 7 years on 37s with the stock 4.10 gears. I just drive it like a 4-speed. 1st is low enough to get going even with a 7k lb trailer (driven very VERY carefully) and 4th is basically overdrive
I hate the spacing of 2-3 and 3-4 but re-gearing would just be for the fun of using 5th and 6th. No other incredibly-tangible benefit

 

[GabeBoyTheGreat]

I hate the spacing of 2-3 and 3-4 but re-gearing would just be for the fun of using 5th and 6th. No other incredibly-tangible benefit

Actually, regearing does help with those lower gears. That is exactly why I would go with 4.88s on 35s instead of 4.56 on 35s, which is what I had in the past.

The higher gears have much tighter spacing. If you bring 6th gear down to where 4th gear is now, then if you need to downshift, you now have more options. You can go down to 5th, 4th, or even 3rd if you really need to. Where you are at now, you can only go down to 3rd, but you can't really go down to 2nd.

A close ratio tranny is much more forgiving when it comes to gearing, which is why a lot of 8AT folks don't regear.

 

[azwjowner]

Well, I've got 37" tires with 4.10's. The guy at McLeod basically said "37's, he should be on 5.13's!" to which my Mechanic replied "Yes, but It's a daily driver he drives on the highway. 4.88's would probably be top". I believe the guy from McLeod said most of the users they had worked with that experienced a failure were 4.56 or lower. He made the point that if Jeep is doing the XR package with 4.88's and 35's w/ the manual transmission and V6 (per the builder on the website) and that is considered "optimal" by the OEM, then it would end up having to be a numerically higher (lower) gear ratio for 37's.

I don't know the mathematics and engineering behind it, but the logic checks out i guess.

That's nonsense. Jeep had 4.88s as an option with 33 inch tires too.

Actually, regearing does help with those lower gears. That is exactly why I would go with 4.88s on 35s instead of 4.56 on 35s, which is what I had in the past.

The higher gears have much tighter spacing. If you bring 6th gear down to where 4th gear is now, then if you need to downshift, you now have more options. You can go down to 5th, 4th, or even 3rd if you really need to. Where you are at now, you can only go down to 3rd, but you can't really go down to 2nd.

A close ratio tranny is much more forgiving when it comes to gearing, which is why a lot of 8AT folks don't regear.

Yes. I had documented this for my situation here: https://www.jlwranglerforums.com/forum/threads/manual-transmission-gearing.125303/post-2617752

 

[RubiSc0tt]

The other interesting thing to me was when I said "I never had to worry about this with my TJ's", my mechanic agreed as he had an XJ on 35's with 4.88's in it. My guess is because those older Jeeps were overbuilt for the size of the vehicle, whereas the JL's are engineered to more "modern SUV" standards to hit mileage performance?

 

[Toycrusher]

Actually, regearing does help with those lower gears. That is exactly why I would go with 4.88s on 35s instead of 4.56 on 35s, which is what I had in the past.

The higher gears have much tighter spacing. If you bring 6th gear down to where 4th gear is now, then if you need to downshift, you now have more options. You can go down to 5th, 4th, or even 3rd if you really need to. Where you are at now, you can only go down to 3rd, but you can't really go down to 2nd.

A close ratio tranny is much more forgiving when it comes to gearing, which is why a lot of 8AT folks don't regear.

Touché!

To be fair, I don't have to contend with elevation changes where I live so I can get away with limited gears to choose from. Towing in the mountains, my setup would be miserable.

 

[GabeBoyTheGreat]

Yes. I had documented this for my situation here: https://www.jlwranglerforums.com/forum/threads/manual-transmission-gearing.125303/post-2617752

That is an excellent explanation. Lower axle gears do not change the RPM gap between gears but it does change the speed gap between gears.

The other interesting thing to me was when I said "I never had to worry about this with my TJ's", my mechanic agreed as he had an XJ on 35's with 4.88's in it. My guess is because those older Jeeps were overbuilt for the size of the vehicle, whereas the JL's are engineered to more "modern SUV" standards to hit mileage performance?

I am not sure what you are asking here? 4.88s on 35s still would have required a regear on a TJ or XJ. The lowest gears available on a factory TJ was 4.10s.

Also, keep in mind that 6th gear in the JL is higher than 5th gear in the TJ/XJ, so technically, the JL would require a lower axle ratio to be at the same RPM. This is why the JL is more forgiving when it comes to lower axle ratios because it has a higher overdrive.

 

[RubiSc0tt]

I am not sure what you are asking here? 4.88s on 35s still would have required a regear on a TJ or XJ. The lowest gears available on a factory TJ was 4.10s.

Also, keep in mind that 6th gear in the JL is higher than 5th gear in the TJ/XJ, so technically, the JL would require a lower axle ratio to be at the same RPM. This is why the JL is more forgiving when it comes to lower axle ratios because it has a higher overdrive.

My point was I only ever ran 4:10's and 33" tires in my 04 Rubi and never really noticed any differences, driving or wheeling. Never even considered regearing, and that's why I never considered it until later on my JLUR.

But the point about the difficulty shifting is correct, now that I think more about it.

 

[_olllllllo_]

I attended the 2026 Overland Expo in Flagstaff in May this year. Centerforce had a booth at the event and I spoke with them at length about the issue. They recommended vacuum bleeding the system using the adapter on the reservoir. The advantage with the hydraulic throwout bearing is that bleeding pulls air from the entire system, the slave cylinder allows air to be pulled through and it is a straight shot to the hydraulic throw out bearing. Adjusting the neutral gate can also help fine tune the gear actuation. I have about 15,000 miles on the hydraulic throw out bearing and it is still fantastic compared to the OEM solution.

 

[OldGuyNewJeep]

I attended the 2026 Overland Expo in Flagstaff in May this year. Centerforce had a booth at the event and I spoke with them at length about the issue. They recommended vacuum bleeding the system using the adapter on the reservoir. The advantage with the hydraulic throwout bearing is that bleeding pulls air from the entire system, the slave cylinder allows air to be pulled through and it is a straight shot to the hydraulic throw out bearing. Adjusting the neutral gate can also help fine tune the gear actuation. I have about 15,000 miles on the hydraulic throw out bearing and it is still fantastic compared to the OEM solution.

I'm at 10k on my hydraulic throw-out and agree it is still fantastic compared to OEM (and ACT).

Interesting tidbit about vacuum bleeding via the reservoir. I did PLENTY of that when I had the ACT, but the mechanic who installed my Centerforce said not to do that, and that the CF instructions say gravity bleed (which is what he did, overnight).

I guess this is the part I'm not following: "the slave cylinder allows air to be pulled through and it is a straight shot to the hydraulic throw out bearing"

Can you add more detail?

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