Regear for Ecodiesel - What Ratio

Drooler

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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.
Does this take into consideration installation of:
A. Steel skid plates
B. Winch
C. Rock sliders
D. Recovery gear, compressor, etc.

Thankfully I don’t have over landing gear or a fridge, etc.

Am kinda sad the ecodiesel doesn’t have the 4.10 as an option.





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Does this take into consideration installation of:
A. Steel skid plates
B. Winch
C. Rock sliders
D. Recovery gear, compressor, etc.

Thankfully I don’t have over landing gear or a fridge, etc.

Am kinda sad the ecodiesel doesn’t have the 4.10 as an option.
No, it doesn't include any other weight or outfitting considerations. All of those figures were just based on gear ratios. If you're adding a lot of weight - either for overlanding or for protection - you'll probably want to consider one "step" lower in your gear ratio. (i.e. 4.56 instead of 4.10, or 4.88 instead of 4.56)
 

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I’ve got 3.55 gears in a 2014 Ecodiesel RAM. Early-on, I wished sometimes that I’d gotten the 3.73 because it took only a minor headwind or slope to make it downshift to 7th. Then got the GDE tune. Not only is the mpg and throttle response better, but it holds 8th gear with a noticeably greater load than before, to the point I quit wishing I’d gotten the 3.73.

When the GDE emission-compliant tune arrives next year for the Wrangler (I don’t know that, but am hopeful), I’m thinking it may make a similar difference in available torque around that 1500rpm range, to the point that I’ll quit thinking about 3.73 vs 4.10 when running 35” tires.

What I don’t know is if having to be epa-compliant will prevent that performance improvement?
 

rcr1340

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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.
I just looked it up and Aussie lockers are only available for the M186(d30). The diesels all come with a m210(D44). Just giving you a heads up bud
 

40”JLURD

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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.
If you’re still waiting on the Dynatrac axles I would try to change the ratios to 5.38 if you can at all. I ran a gear calculator for you and 5.38 with 42.5” tires would be nearly an identical match to 37” and 4.56 which seems to be the preferred combination everyone loves on these diesel motors.

By comparison the 5.13 with 42.5” only provides a tiny power bump over stock 3.73/32” tire combination. It’s almost an identical match only off by 2mph at 2000 rpm in 8th gear.

With how big/heavy those 42.5” meats are gonna be I am certain you will be much happier with going 1 R&P set lower geared than stock - or more - but 1 deeper than stock would be the 5.38
 
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So I can report my 4.56 are I. And running great. Have done my 500 miles of break in (sucks running 500 miles in 4HI! Ugh!).

Now issues during the post break in oil change. Went wheeling on them today and had AMAZING power! Also have gained some decent MPG when highway running! 2-4 MPG additional depending on how leadfooted I am driving.

my Yukon 4.56 gears are fantastic now if I can just get my ARB twin in so I can use the front locker I had installed during the gear swap!
 

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I'm sure folks are thinking about this, but I'll pipe in anyway. Are you all considering the vast amount more torque being applied by the diesel? 442 lb ft is more than my '93 Dodge/Cummins put out new (400 lb ft) , and it had Dana 60/70 axles. By most accounts, a 37" tire is the biggest you should run on a Dana 44. But that "rule of thumb" has been based on gas engines with typically much less torque output (transplanted engines not withstanding). For me, I'm sticking to 35's mostly to reduce the increased leverage on the factory axles when wheeling. I will keep the 3.73 gears because as you go "lower", the pinion gears get smaller (diameter & tooth size), and is therefore more apt to fracture under stress loading (i.e., 37" tires up against a rock), not to even mention the front axle u-joints, which typically quit before the pinion. My '72 CJ-buggy runs a 44/60 axle combo with 36" IROKS, and it runs the Fordyce Creek Trail with no problem and a little rock stacking, so tire height isn't a concern for me.
 

RafterCExplorations

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I'm sure folks are thinking about this, but I'll pipe in anyway. Are you all considering the vast amount more torque being applied by the diesel? 442 lb ft is more than my '93 Dodge/Cummins put out new (400 lb ft) , and it had Dana 60/70 axles. By most accounts, a 37" tire is the biggest you should run on a Dana 44. But that "rule of thumb" has been based on gas engines with typically much less torque output (transplanted engines not withstanding). For me, I'm sticking to 35's mostly to reduce the increased leverage on the factory axles when wheeling. I will keep the 3.73 gears because as you go "lower", the pinion gears get smaller (diameter & tooth size), and is therefore more apt to fracture under stress loading (i.e., 37" tires up against a rock), not to even mention the front axle u-joints, which typically quit before the pinion. My '72 CJ-buggy runs a 44/60 axle combo with 36" IROKS, and it runs the Fordyce Creek Trail with no problem and a little rock stacking, so tire height isn't a concern for me.
And historically (going back to the old Dick Cepek catalogs in the 80's), the correction formula to maintain the "factory power" is: "new tire diameter/old tire diameter x current axle ratio = new axle ratio needed". So for my needs it would be 35/33 x 3.73 = 3.95. Not enough change to be worth the cost for me.
 
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And historically (going back to the old Dick Cepek catalogs in the 80's), the correction formula to maintain the "factory power" is: "new tire diameter/old tire diameter x current axle ratio = new axle ratio needed". So for my needs it would be 35/33 x 3.73 = 3.95. Not enough change to be worth the cost for me.
My original tires were 32’. Original gears were 3.73. Current tires ar 37’s.

So 37/32 x 3.73 = 4.3128. I think that warrants my gear swap to 4.56. Also mine were free so why not do it.
 

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My original tires were 32’. Original gears were 3.73. Current tires ar 37’s.

So 37/32 x 3.73 = 4.3128. I think that warrants my gear swap to 4.56. Also mine were free so why not do it.
Well if you want to get specific.... Your factory Rubicon tires are 32.7" tall at about 40 psi and most "37's" are actually about 36.8" tall at 40 psi. So.... (36.8/32.7) x 3.73 = 4.1976 (4.2), so your 4.56 gears will definitely get you plenty of power. It will also take that 442 lbs ft of torque and increase the load on your axles by 1.25x more than from the factory. I wish knew how to show the increased "torque arm" load put on the axles by 37" tires vs 32's, but I know its significant.
Your rig is your rig, and I hope it works great for you, but these diesels are a completely different animal than the relatively weak/mild 3.6 V6. Their potential to do drivetrain damage when built up is much greater. For me, I'll proceed cautiously.
 

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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.
Wow I wonder what's up with your Jeep? My Ecodiesel will grab 8th at just over 60 on flat ground. And it will hold 8th at 75mph even on slight inclines. I'm running 37x12.50 KM3's with a 2" lift, stock gearing. Seems strange to me you're not cruising in 8th.
 
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Wow I wonder what's up with your Jeep? My Ecodiesel will grab 8th at just over 60 on flat ground. And it will hold 8th at 75mph even on slight inclines. I'm running 37x12.50 KM3's with a 2" lift, stock gearing. Seems strange to me you're not cruising in 8th.
Dunno but I can tell you I am in 8th gear at about 50 mph now so I feel my gear change was a win in my situation at least.....
 

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40”JLURD

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So I can report my 4.56 are I. And running great. Have done my 500 miles of break in (sucks running 500 miles in 4HI! Ugh!).

Now issues during the post break in oil change. Went wheeling on them today and had AMAZING power! Also have gained some decent MPG when highway running! 2-4 MPG additional depending on how leadfooted I am driving.

my Yukon 4.56 gears are fantastic now if I can just get my ARB twin in so I can use the front locker I had installed during the gear swap!
So what’s your average highway MPG now with the gears and 37’s?
 

xtremejoe

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Thanks everyone for the commentary. I wouldn't mind getting feedback from folks that are have been running 37s and 4.56 or 4.88 whether they are happy and what style of driving they do (DD, overland or crawling).
I have had diesels before (SUV and big truck), but the Jeep is a different animal and I'm doing a different ride with this one as a full overland build on my JL. Thx!
 

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