Regear for Ecodiesel - What Ratio

40”JLURD

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4.56 for 37’s and 4.88 for 40’s on a diesel





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40”JLURD

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40”JLURD

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Are the numbers in red on my image the MPH where the gearbox will shift into 8th gear?

Thank you very much for this information! That is amazing!!!

I am heading over to read the thread now.

-DD
No it has nothing to with shifting. It’s the MPH at 2000 RPM. In the red circle you made you would only me looking at the MPH in GREEN column on the right.

This is what your MPH would be with 37” tires and going 2,000 RPM in 8th gear - 71 MPH - This is extremely close to stock and actually puts you ever so slightly better into the powerband of the diesel. So you’ll theoretically get better MPG and have more power on tap VS stock.

In the pic I posted of my gauge cluster I’m bone stock going 76 MPH @ 2,000 RPM in 8th gear.
 

Col_Sanders

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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.

You won my regear! I dumped a lot of tickets into that one, the Clayton lifts, and Rockslide Sliders. At least I won something though (Heatbeaters Headliner kit).

I have a Spartan lunchbox locker in the front of my XJ and IMO if you can swing it, get a selectable...The ratcheting and popping on turns, especially tight turns can get annoying at times. I get some weird looks in the parking lot sometimes.
 
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You won my regear! I dumped a lot of tickets into that one, the Clayton lifts, and Rockslide Sliders. At least I won something though (Heatbeaters Headliner kit).
Sorry not Sorry? :)

I have a Spartan lunchbox locker in the front of my XJ and IMO if you can swing it, get a selectable...The ratcheting and popping on turns, especially tight turns can get annoying at times. I get some weird looks in the parking lot sometimes.
I did some research on them. I know about the clicking and popping. I cannot afford a selectable locker right now so that is why I am going with the lunchbox style. Also my research indicates that the clicking and popping is not usually that bad when you only have it in the front. Which axle is yours in?

-DD
 

Col_Sanders

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Sorry not Sorry? :)



I did some research on them. I know about the clicking and popping. I cannot afford a selectable locker right now so that is why I am going with the lunchbox style. Also my research indicates that the clicking and popping is not usually that bad when you only have it in the front. Which axle is yours in?

-DD

No, I'm happy for ya...At least you can use them, unlike the guy across from me who won a Yukon JLR diff cover and didnt even own a JL. He wouldnt even give me a good deal on it. lol

Mine is in the front. It may not be as bad as it would be in the rear, but is for sure noticeable, especially sharp turns on high traction surfaces. Most of the time it is just a light ratcheting noise but every now and then it will be a bit louder pop or three. Price is the reason I put one in the XJ (and why its still in there) but after having selectable lockers, I would not want to go back to a lunchbox on a street driven rig. If they'll give you a little time to save up for it, you'd be glad you did. I'm sure labor would be much less doing it at once than having them open it up again later.
 
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No, I'm happy for ya...At least you can use them, unlike the guy across from me who won a Yukon JLR diff cover and didnt even own a JL. He wouldnt even give me a good deal on it. lol

Mine is in the front. It may not be as bad as it would be in the rear, but is for sure noticeable, especially sharp turns on high traction surfaces. Most of the time it is just a light ratcheting noise but every now and then it will be a bit louder pop or three. Price is the reason I put one in the XJ (and why its still in there) but after having selectable lockers, I would not want to go back to a lunchbox on a street driven rig. If they'll give you a little time to save up for it, you'd be glad you did. I'm sure labor would be much less doing it at once than having them open it up again later.
Yeah they are not charging me for labor at all since they will already have it open.

-DD
 

Mr. Curti

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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.
When you run your calculations, what do you come up with if a diesel is on 43’s?
 

MCJA

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When you run your calculations, what do you come up with if a diesel is on 43’s?
Assuming true 43" diameter, and not taking into consideration mass, here are the numbers:
  • 4.56 = -7.53% torque and MPG, +7.53% RPMs
  • 4.88 = -0.45% (this is even numerically closer than 40s on 4.56s, at +0.57%)
  • 5.13 = +4.42%
  • 5.38 = +8.86%
That said, with that much rolling mass, you'll probably want the 5.38s. Anything less than that and you'll probably feel a difference. Remember that tire mass goes up exponentially while ratios increase only linearly. (That whole pi*r^2 thing)

@AnnDee4444 and I posted two different versions of a gearing calculator here: https://www.jlwranglerforums.com/forum/threads/gearing-calculator.38206/

Two different approaches to the same stuff; the results should confirm each other. (One shows speed at RPM, the other shows RPM at speed.)
 

Mr. Curti

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Assuming true 43" diameter, and not taking into consideration mass, here are the numbers:
  • 4.56 = -7.53% torque and MPG, +7.53% RPMs
  • 4.88 = -0.45% (this is even numerically closer than 40s on 4.56s, at +0.57%)
  • 5.13 = +4.42%
  • 5.38 = +8.86%
That said, with that much rolling mass, you'll probably want the 5.38s. Anything less than that and you'll probably feel a difference. Remember that tire mass goes up exponentially while ratios increase only linearly. (That whole pi*r^2 thing)

@AnnDee4444 and I posted two different versions of a gearing calculator here: https://www.jlwranglerforums.com/forum/threads/gearing-calculator.38206/

Two different approaches to the same stuff; the results should confirm each other. (One shows speed at RPM, the other shows RPM at speed.)
We ended up going with 5.13’s. With so much unknown with the diesel, we be gambling. Hopefully it don’t bite us to badly.
 

Mr. Curti

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Lots of folks will want to hear what your experience is. 43" tires - that's a LOT of tire.
the front doesn’t look like it’s going to be a problem. The back is going to be a little tricky. The location of the DEF tank is far from ideal and the fuel tank being pushed so far back is going to require a little innovation and ingenuity. Yet we have a plan that we feel will work.

Hopefully the Dynatrac axles and King coilover’s will show up next week so we can start performing surgery. And as for the gearing, we’ve got our fingers crossed. Otherwise we are going to have to swap the 5.13’s for 5.38’s.

Rest asunder we will have a lot of pictures, videos and a solid review on what works, doesn’t work and needs to be changed. This is probably the last toy I’m going to ever own, so we are throwing everything at it to build up the ultimate JL diesel seen thus far.
 

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