Why should I regear?

[mgroeger]

The sport and sahara models come with 32s and 3.45 gears. Up until 2024 the willys had 33s and the same 3.45s. If the 32s and 3.45s are in the sweet spot then 4.10s mathematically are the sweet spot for 38s. Even if you you factor in the extra weight 37s will run around pretty similarly to a stock sport. The idea that is so critical that you need to match it to the stock tire vs gearing is easily disproven by the wide variety of "stock" ratios. Conversely the xr package comes with 4.56s and 38s and 4.95 axle gears. Where exactly is the sweet spot?

@grimmjeeper knows way more than me and said it best. I should have noted

I agree, but with the caveat of not going too deep on the gears. I figured if I was spending the money I'd go all the way. 5.38s and 38s was the worst gearing combo I've run with the 8 speed and there's been a lot of them. It was in 8th by 40mph and lost 2-3mpg on freeway roadtrips where we set the cruise control at 85mph regularly. And with a Rubicon and the 4-1 transfer case 1st gear was almost useless in 4LO. My favorite combo was 4.56s and 37s for freeway speeds and all having plenty of crawl ratio in 4LO. That's such a small difference though I'll likely never regear the JT mojave as the 4.10s and 37s aren't bad and it regularly sees the 100mph limiter out in the desert so I don't need more gearing. More hp would be nice and if the 3.6L kicks the bucket I might do a 6.4L swap or the LT3. Definitely won't need more gearing with 500+ hp.

One thing don't understand though is the people who say they never see 8th gear if they don't re-gear. I have had 4.10s and 38s, 5.13s and 38s, 4.10s and 37s and always saw 8th on highway and even around town and could crawl my brains out in 1st gear beautifully.

Side note: Referring to my post about the sweet spot... My understanding was the transmission is working out of its optimum zone when the gears aren't proper for the tire size, thus leading to more wear and tear on the tranny.

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[Zandcwhite]

@grimmjeeper knows way more than me and said it best. I should have noted


One thing don't understand though is the people who say they never see 8th gear if they don't re-gear. I have had 4.10s and 38s, 5.13s and 38s, 4.10s and 37s and always saw 8th on highway and even around town and could crawl my brains out in 1st gear beautifully.

Side note: Referring to my post about the sweet spot... My understanding was the transmission is working out of its optimum zone when the gears aren't proper for the tire size, thus leading to more wear and tear on the tranny.

The optimum zone is the part I question though. With the factory providing 32s with 3.45s or 3.73s (diesel), 33s with 3.45s, 3.73s, or 4.10s, and 35s with 4.56s as well as the option for 4.88s with 33s, that's a whole range of "optimal". The 8 speed was designed to shift a lot with its 2 overdrives and super deep 1st gear so I doubt there's any extra wear and tear regardless of gear choice.

 

[entropy]

The sport and sahara models come with 32s and 3.45 gears. Up until 2024 the willys had 33s and the same 3.45s. If the 32s and 3.45s are in the sweet spot then 4.10s mathematically are the sweet spot for 38s. Even if you you factor in the extra weight 37s will run around pretty similarly to a stock sport. The idea that is so critical that you need to match it to the stock tire vs gearing is easily disproven by the wide variety of "stock" ratios. Conversely the xr package comes with 4.56s and 38s and 4.95 axle gears. Where exactly is the sweet spot?

Just a small correction. The sports were closer to 31.5s. The pre 2024 willys were 32s, a bit smaller than the 33s on the rubicons.

 

[grimmjeeper]

I agree, but with the caveat of not going too deep on the gears. I figured if I was spending the money I'd go all the way. 5.38s and 38s was the worst gearing combo I've run with the 8 speed and there's been a lot of them. It was in 8th by 40mph and lost 2-3mpg on freeway roadtrips where we set the cruise control at 85mph regularly. And with a Rubicon and the 4-1 transfer case 1st gear was almost useless in 4LO. My favorite combo was 4.56s and 37s for freeway speeds and all having plenty of crawl ratio in 4LO. That's such a small difference though I'll likely never regear the JT mojave as the 4.10s and 37s aren't bad and it regularly sees the 100mph limiter out in the desert so I don't need more gearing. More hp would be nice and if the 3.6L kicks the bucket I might do a 6.4L swap or the LT3. Definitely won't need more gearing with 500+ hp.

Absolutely you can go too far.

It would be a good idea to use an online gear ratio calculator before settling on a specific ratio just as a sanity check.

There's a fe good ones out there. One is really pretty awesome and it lets you do a side-by-side comparison.

 

[grimmjeeper]

The optimum zone is the part I question though. With the factory providing 32s with 3.45s or 3.73s (diesel), 33s with 3.45s, 3.73s, or 4.10s, and 35s with 4.56s as well as the option for 4.88s with 33s, that's a whole range of "optimal". The 8 speed was designed to shift a lot with its 2 overdrives and super deep 1st gear so I doubt there's any extra wear and tear regardless of gear choice.

The diesel only came with 3.73 because the front M210 doesn't have a taller ratio available. It would have been better with 3.45 or even taller for stock tires. But the front M186 wasn't strong enough to handle the torque.

 

[mgroeger]

Absolutely you can go too far.

It would be a good idea to use an online gear ratio calculator before settling on a specific ratio just as a sanity check.

There's a fe good ones out there. One is really pretty awesome and it lets you do a side-by-side comparison.

I found these, are any of these the "pretty awesome" one?

https://differentials.com/technical-help-2/differential-gear-ratio-calculator/
https://www.strangeengineering.net/gear-ratio-calculator/
https://www.currieenterprises.com/c...qKpLmVVaU0-_uH9jBGk7AoGrQ8nfGqjb54BX3h2Ha9zMr
https://www.richmondgear.com/gear-transmission-calculators/

 

[grimmjeeper]

I found these, are any of these the "pretty awesome" one?

https://differentials.com/technical-help-2/differential-gear-ratio-calculator/
https://www.strangeengineering.net/gear-ratio-calculator/
https://www.currieenterprises.com/c...qKpLmVVaU0-_uH9jBGk7AoGrQ8nfGqjb54BX3h2Ha9zMr
https://www.richmondgear.com/gear-transmission-calculators/

I've heard that there's one that's better than those.

 

[roaniecowpony]

Thanks for the suggestion. I hadn’t thought of Rubi take off axles. Though that doesn’t answer my original questions.

The answer is: You shouldn't... until you find out enough about what it changes and how that can benefit you. Then you should weigh the improvement vs the cost. Only you will be able to weigh the cost vs benefit, because spending the money is a different "cost" to people with different resources.

 

[Blckdog]

Joining the conversation with a scenario; 2023 JLU Willys equipped with 3.6 etorque, 8 sp auto, 3.45 gears and the 255 75 17 Firestone MT’s.
I spend my wheeling time on the beach.

Here’s the question, I want to put either 315 70 17s on or 35 12.5 17 tires on. I will likely change wheels to an 8.5 width , but for now will use my 7.5 OEM Moab’s with spacers. It’s a Jeep so mileage will be what it is. So that is not a major factor to me. But I don’t want to screw things up mechanically either.
For the sake of this conversation, recommend a gear set up and reasons why you chose that set up.

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