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Gearing for 37s to 40s??

JC JT

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Hello All,
I have been in the diesel world before Diesels where even cool.
With that said i was a avid Cummins guy just sold my completely restored 1997 12V 4x4 to buy a new Rubicon Diesel.

My order just came in WHite Rubicon Loaded with everything you could get.
Took it home pulled all the stock suspension out and replace it with a 3.5 TeraFlex IR Full system.
Went with Nitto Terra Grapplers 37/12.50/17s
Here is my Question:
I want to regear to bring it back to stock
From what I know 373 gear needs to come up to a 410 gear to offset the 285/70/17 difference of the 37/12.50/17.
I want to know if anyone has regeared a Diesel JT or JL yet.

Before anyone says you don't need to please don't. I know to get it back in its TQ curve it needs more gear.
Thank you for your help
V/r JC
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Stevevdbh

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Hello All,
I have been in the diesel world before Diesels where even cool.
With that said i was a avid Cummins guy just sold my completely restored 1997 12V 4x4 to buy a new Rubicon Diesel.

My order just came in WHite Rubicon Loaded with everything you could get.
Took it home pulled all the stock suspension out and replace it with a 3.5 TeraFlex IR Full system.
Went with Nitto Terra Grapplers 37/12.50/17s
Here is my Question:
I want to regear to bring it back to stock
From what I know 373 gear needs to come up to a 410 gear to offset the 285/70/17 difference of the 37/12.50/17.
I want to know if anyone has regeared a Diesel JT or JL yet.

Before anyone says you don't need to please don't. I know to get it back in its TQ curve it needs more gear.
Thank you for your help
V/r JC
Not trying to talk you out of it but waiting for my JTRD and reviewing the same question. THis was helpful for me:
 

WXman

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Hello All,
I have been in the diesel world before Diesels where even cool.
With that said i was a avid Cummins guy just sold my completely restored 1997 12V 4x4 to buy a new Rubicon Diesel.

My order just came in WHite Rubicon Loaded with everything you could get.
Took it home pulled all the stock suspension out and replace it with a 3.5 TeraFlex IR Full system.
Went with Nitto Terra Grapplers 37/12.50/17s
Here is my Question:
I want to regear to bring it back to stock
From what I know 373 gear needs to come up to a 410 gear to offset the 285/70/17 difference of the 37/12.50/17.
I want to know if anyone has regeared a Diesel JT or JL yet.

Before anyone says you don't need to please don't. I know to get it back in its TQ curve it needs more gear.
Thank you for your help
V/r JC
The problem with that line of thinking is that you are assuming that they are using the appropriate gear for the power band to begin with. Unfortunately they are not. This engine makes peak torque at 1500 RPM. It functions best with 3.21 gears if using stock tires. That's the standard ratio in the Ram. Not sure why they didn't do the same in the Jeep. Fuel efficiency drops off FAST with the deeper axle ratios. I know because I've experienced it with this engine all year. The extremely narrow power band in the diesel makes it even more critical to keep the RPMs as low as possible. Personally I'd never regear this powertrain.
 
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JC JT

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Thank you both for the help.
Just trying to wrap my head around the power and TQ.
I am thinking of my Ram Diesels they performed best with 410s and 35 Ats
thought this would be no different.
Guess I need to get the TAZER on it and set the Speed & TIre size to see how it really is doing
 

JTRdiesel

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The problem with that line of thinking is that you are assuming that they are using the appropriate gear for the power band to begin with. Unfortunately they are not. This engine makes peak torque at 1500 RPM. It functions best with 3.21 gears if using stock tires. That's the standard ratio in the Ram. Not sure why they didn't do the same in the Jeep. Fuel efficiency drops off FAST with the deeper axle ratios. I know because I've experienced it with this engine all year. The extremely narrow power band in the diesel makes it even more critical to keep the RPMs as low as possible. Personally I'd never regear this powertrain.

To add to this, i'm planning on running 40's with some XD60s and i'm planning on no lower than a 4.10 gearset, this puts me at 1520rpm at 65mph (105KM/h) on the highway
 
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Thanks for starting this thread. It will help me decide which gears to order the diesel with from the get go. I will more than likely have 37s with an Alu-Cab Canopy Camper, Longer Ranger Aux tank if ever available, and at least 20+ gallons of permanent water.....plus all the normal gear and food.
 

Stevevdbh

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Thank you both for the help.
Just trying to wrap my head around the power and TQ.
I am thinking of my Ram Diesels they performed best with 410s and 35 Ats
thought this would be no different.
Guess I need to get the TAZER on it and set the Speed & TIre size to see how it really is doing
Folowing
 

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Regearing has been covered extensively here
 

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I guess I should add that 40" and larger tires are huge. That's RARE to see on a Jeep, especially with stock axles. If that's your plan, that totally changes the ballgame.

But on 35s or 37s, the stock 3.73 ratio will put you right in the sweet spot for highway RPM. I'm running 34" tires on 3.92 gears with this engine and trans and it's overkill unless I'm loaded all the way up and towing. But just in daily driving it's definitely overkill. I can't break 22 MPG. The guys with 3.21 gears are seeing mid to upper 20s.
 

JTRdiesel

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I guess I should add that 40" and larger tires are huge. That's RARE to see on a Jeep, especially with stock axles. If that's your plan, that totally changes the ballgame.

But on 35s or 37s, the stock 3.73 ratio will put you right in the sweet spot for highway RPM. I'm running 34" tires on 3.92 gears with this engine and trans and it's overkill unless I'm loaded all the way up and towing. But just in daily driving it's definitely overkill. I can't break 22 MPG. The guys with 3.21 gears are seeing mid to upper 20s.
i totally agree with you that changing ratios below 37's would be mostly pointless if you want to drive around at peak or just before peak torque on this engine on the highway. 38's and 40's are a whole different ball game, and imho 4.10s will be the best compromise between more power and Range. I'm taking the plunge and doing 1 tons for overkill safety. i test drove both gear ratios in the Ram and would have gotten 3.92s every day because of the extra power i felt. fuel mileage is nice but with a big tank in the ram its less of a concern, gladiator with a fuel capacity of only about 60% i think the mileage matters a hair more.
 

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So even with a kitted out build, if gas mileage is the goal, you’d stick with the lowest gear ratio offered from the factory?
Do you think the engine would still be able to maintain 65-75 up I-70 at 11,000 feet?
I’m guessing yes.
 

JTRdiesel

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So even with a kitted out build, if gas mileage is the goal, you’d stick with the lowest gear ratio offered from the factory?
Do you think the engine would still be able to maintain 65-75 up I-70 at 11,000 feet?
I’m guessing yes.
i cant speak for Wxman but for me if im going to drop a gear at extreme altitudes, im not worried about it, up that high and climbing isn't where you're going to have much in the way of fuel economy anyway.
i dont have any empircal data to go on ,but i do think that it'll hold 7th on 40's
 
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JC JT

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285/70/17 to 37/12.50/17 + 373 effective gear = 314 gear. So to get back close New ratio 402 equivalent to 410
I will keep all posted of how it drives out with 410s
It's not like its costing me anything, I have my shop and gears are no issue.
I know you all think it's not going to make any difference, but if will be close to factory with 410s
That's my goal, I like trying things to see what works best.
If its a flop I will be honest with all and tell my story.
 

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If tires are 37 or smaller, yes it'll hold speed with stock gearing easily. I can put an ATV in the bed, load the back seat with camping gear, then hook on a 4,500 lb. 9 foot high enclosed trailer and set the cruise at 70 and it'll hold 8th gear, even on long hills. It won't downshift until I hit significant grades. I don't see why a Jeep on 37s wouldn't be able to hold speed with 3.73s.

I know there are guys who want to gear the hell out of these things, but you have to remember that the EcoD makes nearly double the torque and it does so at literally half the RPM. It really is a totally different ballgame compared to the Pentastar.
 
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JC JT

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If tires are 37 or smaller, yes it'll hold speed with stock gearing easily. I can put an ATV in the bed, load the back seat with camping gear, then hook on a 4,500 lb. 9 foot high enclosed trailer and set the cruise at 70 and it'll hold 8th gear, even on long hills. It won't downshift until I hit significant grades. I don't see why a Jeep on 37s wouldn't be able to hold speed with 3.73s.

I know there are guys who want to gear the hell out of these things, but you have to remember that the EcoD makes nearly double the torque and it does so at literally half the RPM. It really is a totally different ballgame compared to the Pentastar.
I agree Gas to Diesel, But I have been in Diesels all my life. So lets see what happens.
That is good to know on your Tow and loading Thank you for that information
Heck once I am done i will drive it down to see you Heck we are close Southern Indiana
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