Are 4.10s sufficient?

[Zandcwhite]

It all depends on what you want to do and how tolerant you are of loss of power. You could re-gear your existing setup a lot cheaper than buying a whole new Jeep, but I get it sometimes you want something new.

You will likely find that 4.10 and 35's are still inadequate depending on elevation, use case, etc.

Generally speaking, this is what I recommend and have done--I re-geared twice:

3.6L/ZF8:

33" = 4.56
35" = 4.88
37" = 5.13
38+ = 5.38

Aside from 1st gear, gearing axles does absolutely nothing to “make up for loss of power”. 4.10 gears in 7th gear have the exact same final drive ratio as 5.13’s in 8th. I’ve yet to drive a JL with an 8 speed regardless of gearing or tire size that felt sluggish off the line, but maybe your experience is different? Having run 4.10’s for tens of thousands of miles on 33’s, 37’s, and 38’s, and then 5.38’s on 38’s I absolutely feel the Jeep was better off geared too high than too low. The mpg was worse, somehow the 0-60 even got slower, and the thing felt like it needed another gear on the freeway. Now with the new XR, even the 4.56’s and 35’s seem a little lower than optimal for me. Not enough to regear, but I think they will be a perfect match for 37’s for my use. We live at nearly at sea level, but wheel well above 10k’ regularly. I’ll never gear another JL in the 5’s unless it’s running 40“+ tires.

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[jmr]

I'm running 285/70-17 (32.8") on my JL Willys w/ 3.45 gears not a big change since stock is 255/75-17 (32.1") so it drives fine.

So what would the tallest tire that would work with the 3.45 gears a 34" ?

 

[CarbonSteel]

Aside from 1st gear, gearing axles does absolutely nothing to “make up for loss of power”. 4.10 gears in 7th gear have the exact same final drive ratio as 5.13’s in 8th. I’ve yet to drive a JL with an 8 speed regardless of gearing or tire size that felt sluggish off the line, but maybe your experience is different? Having run 4.10’s for tens of thousands of miles on 33’s, 37’s, and 38’s, and then 5.38’s on 38’s I absolutely feel the Jeep was better off geared too high than too low. The mpg was worse, somehow the 0-60 even got slower, and the thing felt like it needed another gear on the freeway. Now with the new XR, even the 4.56’s and 35’s seem a little lower than optimal for me. Not enough to regear, but I think they will be a perfect match for 37’s for my use. We live at nearly at sea level, but wheel well above 10k’ regularly. I’ll never gear another JL in the 5’s unless it’s running 40“+ tires.

I totally and adamantly disagree with "Aside from 1st gear, gearing axles does absolutely nothing to “make up for loss of power”". Somehow, you seem to be suggesting that 3.45s and 37's would yield the same driving experience and power ranges as 5.13s and 37's. That would be absolutely incorrect--the axle gearing affects EVERY gear not just 1st. In addition, it is not the lower end of the spectrum that is the problem--it is the upper end and particularly on the highway.

The vast difference between my experience and yours is that I incrementally went up from 4.10 (with both 33's and 35's) to 4.88 (with both 35's and 37's) and finally to 5.13 (with 37's) over a period of 50K miles.

This gave me much more insight into what does and does not work versus your experience of over-gearing from 4.10 to 5.38 from the start. None of the things that you describe have been my experience nor has it been the many others who have commented on my many posts about this.

My Jeep does not feel slow, it does not feel like it needs another gear at highway speeds, and if the speed is below 75, I average between 17-19MPG (depending on headwinds, hills, etc.)

 

[CarbonSteel]

I'm running 285/70-17 (32.8") on my JL Willys w/ 3.45 gears not a big change since stock is 255/75-17 (32.1") so it drives fine.

So what would the tallest tire that would work with the 3.45 gears a 34" ?

Describe "work" and what that translates into and what engine and transmission.

 

[c20040215]

I would totally and adamantly disagree with "Aside from 1st gear, gearing axles does absolutely nothing to “make up for loss of power”". Somehow, you seem to be suggesting that 3.45s and 37's would yield the same driving experience and power ranges as 5.13s and 37's. That would be absolutely incorrect--the axle gearing affects EVERY gear not just 1st.

The vast difference between my experience and yours is that I incrementally went up from 4.10 (with both 33's and 35's) to 4.88 (with both 35's and 37's) and finally to 5.13 (with 37's) over a period of 50K miles.

This gave me much more insight into what does and does not work versus your experience of over-gearing from 4.10 to 5.38 from the start. None of the things that you describe have been my experience nor has it been the many others who have commented on my many posts about this.

My Jeep does not feel slow, it does not feel like it needs another gear at highway speeds, and if the speed is below 75, I average between 17-19MPG (depending on headwinds, hills, etc.)

I love the 80/20 rule. It holds true in many situations. 80% people who give out suggestions/experiences on 8At JL with 35s are 4.56 or 4.88, 37s are 4.88 or 5.13, and 37+ are 5.13 or 5.38. There will be 10% telling you that 4.10 is totally fine regardless the tire size because the 8AT is so magical, and the other 10% will tell you they are on 37s + 5.38 and wish they could go deeper.

I would go with the 80%.

 

[Zandcwhite]

I would totally and adamantly disagree with "Aside from 1st gear, gearing axles does absolutely nothing to “make up for loss of power”". Somehow, you seem to be suggesting that 3.45s and 37's would yield the same driving experience and power ranges as 5.13s and 37's. That would be absolutely incorrect--the axle gearing affects EVERY gear not just 1st. In addition, it is not the lower end of the spectrum that is the problem--it is the upper end and particularly on the highway.

The vast difference between my experience and yours is that I incrementally went up from 4.10 (with both 33's and 35's) to 4.88 (with both 35's and 37's) and finally to 5.13 (with 37's) over a period of 50K miles.

This gave me much more insight into what does and does not work versus your experience of over-gearing from 4.10 to 5.38 from the start. None of the things that you describe have been my experience nor has it been the many others who have commented on my many posts about this.

My Jeep does not feel slow, it does not feel like it needs another gear at highway speeds, and if the speed is below 75, I average between 17-19MPG (depending on headwinds, hills, etc.)

My point was, the only place you actually have a lower gear available than the guy with 4.10's is 1st gear. Beyond that, his 8 speed my be in a gear below yours, but the drive ratio/ load on the engine/ etc will be the same. It's simple math and the 8 speed does a great job of picking the gear needed. In 7th gear, the leverage the engine produces on the wheels via 4.10's is identical to the leverage 5.13's produce in 8th. The only place the 5.13's have more torque available at the wheels is 1st gear, beyond that they obviously produce more torque in a given gear, but that's what downshifts are for anyway. If the load needs say 200 ftlbs at the tire to maintain speed, it makes no difference to the engine if that comes via 8th gear and 5.13's, 7th and 4.10's,or even 6th and 3.45's. The engine needs to be at whatever rpm it needs to be at to produce the required torque and doesn't know or care what gear the trans is in. Not that I'm suggesting 4.10's are optimal for 37's, likely 4.88's are in my experience, but they are more than adequate and absolutely better on the freeway than 5.38's in my experience.

 

[jmr]

Describe "work" and what that translates into and what engine and transmission.

It's a 2.0L with A8. I could be wrong but I think the 32.8" 285/70-17 is probably at the top range for 3.45's for good highway performance and light off road use.

Gears make a huge difference in acceleration. Back in the day I bought a 70 340 Cuda w/ 727 auto that someone had swapped the oem 3.55's for 2.76 gears. I swapped those for 3.91's and it felt like 100 more hp but it was screaming on the highway at 70 mph. Later I found a 3rd member with 3.55's that was a great compromise.

 

[CarbonSteel]

My point was, the only place you actually have a lower gear available than the guy with 4.10's is 1st gear. Beyond that, his 8 speed my be in a gear below yours, but the drive ratio/ load on the engine/ etc will be the same. It's simple math and the 8 speed does a great job of picking the gear needed. In 7th gear, the leverage the engine produces on the wheels via 4.10's is identical to the leverage 5.13's produce in 8th. The only place the 5.13's have more torque available at the wheels is 1st gear, beyond that they obviously produce more torque in a given gear, but that's what downshifts are for anyway. If the load needs say 200 ftlbs at the tire to maintain speed, it makes no difference to the engine if that comes via 8th gear and 5.13's, 7th and 4.10's,or even 6th and 3.45's. The engine needs to be at whatever rpm it needs to be at to produce the required torque and doesn't know or care what gear the trans is in. Not that I'm suggesting 4.10's are optimal for 37's, likely 4.88's are in my experience, but they are more than adequate and absolutely better on the freeway than 5.38's in my experience.

Not following how a guy with 4.10s that the drive ratio/ load on the engine/ etc will be the same. It will not be the same in any gear and yes, it is simple mathematics:

4.10 + 8th = 2.747 final gear ratio
5.13 + 8th = 3.437 final gear ratio

An engine will be working harder under the same load with 4.10s versus 5.13s due to the torque multiplication.

4.88 and 37s are good unless you are at altitude as I am, then with a 3.6L you need more which is why I went to 5.13s.

 

[entropy]

Not following how a guy with 4.10s that the drive ratio/ load on the engine/ etc will be the same. It will not be the same in any gear and yes, it is simple mathematics:

4.10 + 8th = 2.747 final gear ratio
5.13 + 8th = 3.437 final gear ratio

An engine will be working harder under the same load with 4.10s versus 5.13s due to the torque multiplication.

4.88 and 37s are good unless you are at altitude as I am, the with a 3.6L you need more which is why I went to 5.13s.

I think what he is saying is that at a scenario like your example the Jeep with 4.10s will simply downshift to 7 and keep the same ratio. While the jeep with shorter gear will run on 8th speed.

So when the engine goes under load, slight hill, acceleration, the 8speed will downshift accordingly. At the end of the day both Jeeps will drive quite similarly.

The 8 speed transmission does a good job at adapting to whatever gear ratio you have.

 

[Aquastream]

I'm running 285/70-17 (32.8") on my JL Willys w/ 3.45 gears not a big change since stock is 255/75-17 (32.1") so it drives fine.

So what would the tallest tire that would work with the 3.45 gears a 34" ?

I would stay where you are now with the 285/70s.

I had 285/75s(34s) toyo at3s on mine for a short time and it was pretty crappy with 3.45s. Granted i have a v6, the 2.0 is suppose to be much better

 

[CarbonSteel]

I think what he is saying is that at a scenario like your example the Jeep with 4.10s will simply downshift to 7 and keep the same ratio. While the jeep with shorter gear will run on 8th speed.

So when the engine goes under load, slight hill, acceleration, the 8speed will downshift accordingly. At the end of the day both Jeeps will drive quite similarly.

The 8 speed transmission does a good job at adapting to whatever gear ratio you have.

OK; I would agree with that, but it will likely not just downshift to 7th, it will go to 6th or 5th as is what I experienced before I re-geared to 4.88.

 

[Zandcwhite]

Not following how a guy with 4.10s that the drive ratio/ load on the engine/ etc will be the same. It will not be the same in any gear and yes, it is simple mathematics:

4.10 + 8th = 2.747 final gear ratio
5.13 + 8th = 3.437 final gear ratio

An engine will be working harder under the same load with 4.10s versus 5.13s due to the torque multiplication.

4.88 and 37s are good unless you are at altitude as I am, then with a 3.6L you need more which is why I went to 5.13s.

Let's use your math, in a given gear obviously the lower the axle gears the more torque multiplication. 4.10s + 7th gear...3.403 sufficiently identical? The only difference after 1st gear is which gear the trans may be in to produce the rpm required to perform the work, but with one caveat. If the load is low enough, the guy with the 4.10s has that 2.747 8th gear final drive ratio significantly reducing rpms. Will he be in 8th as often as you are? Hell no. Will you ever see as low an rpm as he does under those low load scenarios like coming down a grade at freeway speed? Only if you shift into neutral.

 

[CarbonSteel]

Let's use your math, in a given gear obviously the lower the axle gears the more torque multiplication. 4.10s + 7th gear...3.403 sufficiently identical? The only difference after 1st gear is which gear the trans may be in to produce the rpm required to perform the work, but with one caveat. If the load is low enough, the guy with the 4.10s has that 2.747 8th gear final drive ratio significantly reducing rpms. Will he be in 8th as often as you are? Hell no. Will you ever see as low an rpm as he does under those low load scenarios like coming down a grade at freeway speed? Only if you shift into neutral.

I do not disagree with your thoughts, with a caveat, when under a load (high head winds and hills/mountains), the 4.10 geared vehicle will almost always be in at least two gears lower than 4.88/5.13. I know this to be true because I have lived all of them. You cannot simply assume the 4.10 geared Jeep is just dropping one gear in actual use because odds are it won't--but it all depends on where you are driving the majority of the time which should be a major influencer on your choice of gears.

For example. if you take a 3.6L 4.10/37 and a 3.6L 5.13/37 Jeep and drive from Trinidad, CO to Colorado Springs with a headwind from the north, I know the 4.10 geared Jeep is going to be in 5th and 6th most of that way versus 7th and 8th for the 5.13 which negates the "lower RPM" argument to a large degree.

Here is a sheet I created recently to help a friend with a JK compare to a JL (5 speed versus 8 speed) and you can see the differences between different rear gearing in the JL and different transmission gears.

JK vs. JL Gearing

 

[TJRUBICON06]

I think 2drs. handle 4.10's better than 4 drs. . For several years I ran 37's with 4.10's in my JLR V6 auto. and they worked well for me in lo-range rock crawling. Cost of gear swapping wasn't worth it IMHO. Now I have dropped down to 35's and I am glad to have kept 4.10's and enjoy 20 MPG while pavement pounding 65 MPH. 8th gear is more common on the freeway now when I just leave it in drive, where as I use row back and forth between 6th, 7th and 8th manually.

I agree with you, the extra 500 pounds on a 4 door is noticeable. My wife and I both run 35's on our JLR's. Both automatics. It may be minimal, but the performance seems improved compared to a JLUR.

 

[[DELETED USER 82058]]

Reading through this and assuming this is all about the 4 Door? 2 Door Rubi owners seem happy with 4.10s and 35s.

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