Regear for Ecodiesel - What Ratio

StuartR

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So I won a free regear package for both my front and rear axle's at an event this weekend. I am trying to decide what gear ration I should go to. I can pick anything I want. I currently have 37" tires. Not really expecting to go any bigger for quite some time if ever.

4.88? 5.13? What say you Mind Hive collective? What gears should I pick for my Diesel JL? I do a lot of rock crawler and also pull my boat which is pretty much close to my max weight limit. Looking into getting some sort of trailer that I can pull as well, maybe a popup or something....have to sleep 3 so teardrops are too small for us.





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Compression-Ignition

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any reasons as to why not higher or lower?
Mainly because if I could have factory ordered 4.10's I would have left it alone with probably zero thoughts of a regear. And I think 4.56's is the next step from 4.10's. Like yours, this Jeep will most likely never see anything bigger than 37's.

There is more to it than this, but

3.73 gear ratio / 33" tire = .113
.113 x 36.5" tire = 4.12 gear ratio

Some people will say ^^^ that is nonsense. People can use whatever sort of logic or chart they want. But that makes sense to me. It's all subjective anyway. Shoot most of the guys on the forum are saying they think the stock 3.73's and 37's is a perfect combo. I've said multiple times, IMO the 33's and 3.73's was perfect. After I went to 37's I felt a noticeable decrease in acceleration. It drives fine, but it is no longer ideal......IMO.
 
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StuartR

StuartR

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Mainly because if I could have factory ordered 4.10's I would have left it alone with probably zero thoughts of a regear. And I think 4.56's is the next step from 4.10's. Like yours, this Jeep will most likely never see anything bigger than 37's.

There is more to it than this, but

3.73 gear ratio / 33" tire = .113
.113 x 36.5" tire = 4.12 gear ratio

Some people will say ^^^ that is nonsense. People can use whatever sort of logic or chart they want. But that makes sense to me. It's all subjective anyway. Shoot most of the guys on the forum are saying they think the stock 3.73's and 37's is a perfect combo. I've said multiple times, IMO the 33's and 3.73's was perfect. After I went to 37's I felt a noticeable decrease in acceleration. It drives fine, but it is no longer ideal......IMO.
I like it. I am still open to other opinions but this gives me a good basis for my thought process. Thank you.
 

jdubya421

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If you look at gearing charts for the 3.6L motor it would suggest 4.88 which is what I went with for 37s. With the insane amount of torque in the diesel it might be a little different because I assume like most diesels it doesn't rev nearly as high. If I think about it like a 2500 truck or something, you don't really even need to re-gear when going from the stock 33" to a 35" tire because you just have so much available torque that it is fine. So I might agree with Roy in that 4.56 might be a good option because you will get some of the lost pep and have a larger/stronger ring and pinion vs 4.88 and up.
 
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StuartR

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If you look at gearing charts for the 3.6L motor it would suggest 4.88 which is what I went with for 37s. With the insane amount of torque in the diesel it might be a little different because I assume like most diesels it doesn't rev nearly as high. If I think about it like a 2500 truck or something, you don't really even need to re-gear when going from the stock 33" to a 35" tire because you just have so much available torque that it is fine. So I might agree with Roy in that 4.56 might be a good option because you will get some of the lost pep and have a larger/stronger ring and pinion vs 4.88 and up.

THank you for that insight. It is greatly appreciated.
 

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First, congrats on winning a free gear swap! That's one helluva prize!

Second, I agree with Roy - 4.56 is the sweet spot for 37s and your intended uses.

I've done quite a bit of research on gear/tire combos for the diesel. Hope it helps...

I'll start with just the drive ratio numbers. Not taking into consideration additional rolling mass, increased wind resistance, etc. Just "gearing math".

OEM = 32.7" on 3.73 gears.

37s on 4.10 = 2.27% loss in torque, reduced RPM, increased MPG. For 37" tires specifically, 4.10 gears are numerically the "closest" to OEM tire/gear combo. ("Closest" = absolute value; i.e., least amount of loss or gain.)

37s on 4.56 = 8.05% increased torque, higher RPM, decreased MPG. 8% is a noticeable jump (whether it's a gain or a loss, 8% is quite a chunk)

Interestingly, 40s on 4.56 = 0.57% increased torque, higher RPM, decreased MPG. Mathematically speaking, 40" tires on 4.56 gears are the closest gear/tire combo that match OEM gear/tire. The next closest is 37s on 4.10 at 2.27%, as stated above.

Now, inject some reality into the nerdery. Tire mass matters. Wind resistance matters. I guess one last thing to ask yourself... You're running 37s on the OEM 3.73 gears now, right? What has your experience been? If it's miserably underpowered, then go with lower (numerically higher) gears - 4.56. But if you barely noticed, then probably 4.10s would work. If you plan on towing, the 4.56 will help you avoid "lugging" the engine (struggling at low RPMs). Plus, 4.56 is better for the rocks, which you said you do quite a bit of.

I also agree with what Roy said that anything above 4.56 would be overkill. If we were talking about a gas engine, then 4.88s would be appropriate. But you simply don't need the additional torque, and you really need to avoid the higher RPMs on the diesel.

Bottom line: for the use cases you described, I think 4.56 is the sweet spot.
 
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First, congrats on winning a free gear swap! That's one helluva prize!

Second, I agree with Roy - 4.56 is the sweet spot for 37s and your intended uses.

I've done quite a bit of research on gear/tire combos for the diesel. Hope it helps...

I'll start with just the drive ratio numbers. Not taking into consideration additional rolling mass, increased wind resistance, etc. Just "gearing math".

OEM = 32.7" on 3.73 gears.

37s on 4.10 = 2.27% loss in torque, reduced RPM, increased MPG. For 37" tires specifically, 4.10 gears are numerically the "closest" to OEM tire/gear combo. ("Closest" = absolute value; i.e., least amount of loss or gain.)

37s on 4.56 = 8.05% increased torque, higher RPM, decreased MPG. 8% is a noticeable jump (whether it's a gain or a loss, 8% is quite a chunk)

Interestingly, 40s on 4.56 = 0.57% increased torque, higher RPM, decreased MPG. Mathematically speaking, 40" tires on 4.56 gears are the closest gear/tire combo that match OEM gear/tire. The next closest is 37s on 4.10 at 2.27%, as stated above.

Now, inject some reality into the nerdery. Tire mass matters. Wind resistance matters. I guess one last thing to ask yourself... You're running 37s on the OEM 3.73 gears now, right? What has your experience been? If it's miserably underpowered, then go with lower (numerically higher) gears - 4.56. But if you barely noticed, then probably 4.10s would work. If you plan on towing, the 4.56 will help you avoid "lugging" the engine (struggling at low RPMs). Plus, 4.56 is better for the rocks, which you said you do quite a bit of.

I also agree with what Roy said that anything above 4.56 would be overkill. If we were talking about a gas engine, then 4.88s would be appropriate. But you simply don't need the additional torque, and you really need to avoid the higher RPMs on the diesel.

Bottom line: for the use cases you described, I think 4.56 is the sweet spot.

Thank you for your insight. It is greatly appreciated!!! Honestly I have not felt any real difference in my performance since installing 37's other than now I am in 7th gear at 75 mph on cruise control when I used to be in 8th.

I have not had the opportunity to tow my boat since having the 37's.

I also have steel wheels which certainlyadds to the wheel mass as well so I think 4.56 sounds likeit will be the right place to go.
 

jdubya421

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You will definitely enjoy having gears. It's normally an expensive process but is well worth it, having won it is awesome!
 

MCJA

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Thank you for your insight. It is greatly appreciated!!! Honestly I have not felt any real difference in my performance since installing 37's other than now I am in 7th gear at 75 mph on cruise control when I used to be in 8th.

I have not had the opportunity to tow my boat since having the 37's.

I also have steel wheels which certainlyadds to the wheel mass as well so I think 4.56 sounds likeit will be the right place to go.
Yeah - I don't think you'll be disappointed with 4.56. Our gasser siblings are cringing, because that's probably too high for them. Gotta love 442 lb-ft of FUN.

If you haven't noticed much of a degradation with 37s on 3.73s, then even 4.10s would probably work. But if you're going to tow and crawl, can't go wrong with a bit lower.
 

MCJA

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You will definitely enjoy having gears. It's normally an expensive process but is well worth it, having won it is awesome!
Yeah. I'm a bit jelly, actually.
 

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The diesels are largely unknown still on optimal regear ratio because we’ve never had a factory option with this power. If it was a gas and you’re running 37s, it’s 5.13s all day long.

The worst thing with a diesel is to lug them or run to low of Rpms. My suggestion would be to go with a 4.88 max and 4.56 minimum. I prefer to run a little higher rpm and ensure it’s sitting in 8th on the highway over lugging it or forcing it to run in 7th at higher rpm than needed. The 8 speed has such a deep OD than the rpm difference isn’t that much but the extra power would really Wake the diesel up

congrats on the prize, that’s awesome.

brett
 
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StuartR

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The diesels are largely unknown still on optimal regear ratio because we’ve never had a factory option with this power. If it was a gas and you’re running 37s, it’s 5.13s all day long.

The worst thing with a diesel is to lug them or run to low of Rpms. My suggestion would be to go with a 4.88 max and 4.56 minimum. I prefer to run a little higher rpm and ensure it’s sitting in 8th on the highway over lugging it or forcing it to run in 7th at higher rpm than needed. The 8 speed has such a deep OD than the rpm difference isn’t that much but the extra power would really Wake the diesel up

congrats on the prize, that’s awesome.

brett

Thanks! I appreciate the input.

I called them this morning and started the process for 4.56's. Also having them put in an Aussie (lunchbox) locker in the front axle since they are going to already have it open and have to change the gears out anyway.

Thank everyone for the input. Will let yall know how things work out once the regear is complete.
 

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