Regear for Ecodiesel - What Ratio

[Mr. Curti]

So I have my axles. Started the build then FedEx lost my driveshafts… I went with 5.13. Going to run 40’s for the first set. Next 42’s.

I’m building at a friends shop. While waiting I’ve put over 600 miles on his JTRD running 42’s with 5.38 gearing. Drives close to a stock JTRD around town and up to Freeway speed. Freeway speeds (above 65) would be the only issue for some that do not want to cruise above 2K RPM. Cruise RPM is about the same as a stock 3/4 ton Duramax. I owned many between 2000 and 2010.
This JTRD has over 15k miles. No issues.

Edit. On the return trip from NorCal to AZ. Approximately 500miles. Average speed 70mph. Approximately 2250 RPM. 18 MPG. Speedo correct. GPS verified. JTRD 5.38 gearing. Nitto 42” tires.

Since my post on here, I learned that we ordered 4.88’s. Not sure what happened. Anyhow, we finally got our hands on five 40x15.50R20 Toyo’s. So as soon as they get in, and the shop is cleared of its current project, we can finally (2 years later) get this Jeep built. Now as for the 40’s and 4.88’s, we’ll see how she performs and see if we need to change them.

Patients is a virtue said no Jeep owner! LOL!

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[Mr. Curti]

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

Been a while since I’ve been on here. And I leaned we went up instead of down on our gear ratio. 4.88’s is what we got. Somehow I screwed up. Anyhow, being that the 42’s we ordered will probably never show up and I am tired of this build, we got our hands on Toyo 40’s. So hopefully in the next month or so we will finally have this Jeep built.

 

[Latvia1971]

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.

4.56

 

[SalC]

Thanks to everyone on the thread! I recently regeared my 21 JLURD and went with the 4.56 and it's perfect!

I have the BFG KO2 37x12.50x17 load range C, which run more like 35". My wife has a 21 diesel Sahara Altitude which is a very close match to the stock Rubicon, so I have a control jeep to compare with.

First, the jeep drives exactly like stock, maybe better torque from a stop. At 65 MPH I am only running about 80-100 more RPM than the stock jeep. I see this as headroom for when I get actual 37" tires. Gas millage has returned back to the high 20's.

I'm very happy with the results. I was a little saddened by the performance hit I took putting bigger tires on. This is the best the jeep has ever driven.

 

[Nickmimi]

My recent AEV built Rubicon came with 4.56 gears on 37s..

 

[zouch]

just to clarify; you reference a "recent" Rubi, but your sig shows a 2019.

can you confirm this new AEV rig is a Diesel?

My recent AEV built Rubicon came with 4.56 gears on 37s..

 

[Nickmimi]

just to clarify; you reference a "recent" Rubi, but your sig shows a 2019.

can you confirm this new AEV rig is a Diesel?

Sorry I haven't updated my profile I traded the 2019 for a 2021 AEV JL370 Rubicon diesel

 

[SalC]

Sorry I haven't updated my profile I traded the 2019 for a 2021 AEV JL370 Rubicon diesel

It probably would have been cheaper for me to buy the JL370 instead of going the route I did. Next time maybe.

 

[Nickmimi]

That is how I justified it to myself as well. I added up all of the mods done in my 2019 then relooked at the price with all of the mods that were already done and it made sense. Of course I still added things I took of the old Jeep and some new mods.

 

[Knight Offroad]

I figured I'd chime in with my experiences. I have Fusion 60 axles front and rear with 40s and went with 4.56 based on the majority consensus I had read around the internet. While I'm not unhappy, I think for me, 4.88 would have been better. One ton axles add a tremendous amount of weight. The Maxxis RAZR tires are also very heavy and aggressive. They are almost the full 40" in size. I'm carrying a full size spare, steel tire carrier and of course a winch. I think I added somewhere around 1200 extra pounds all said and done. That's a LOT of weight. I usually only see 8th gear on the interstate but as soon as the road tilts up, even a little, it drops into 7th. At least it doesn't hunt down into 6th.

I think with 4.88 I would have been able to ride most of the rollers out in 8th and picked up a little extra spunk off the line. I will probably address this by changing to the BFG KM3 39s the next time I need tires. That should make the gearing perfect then.

Also, at 80 MPH I'm seeing oil temps between 230-240. That's in the rollers. Once I hit the bigger mountain passes I'm going to have to slow way down.

If you are doing 40s and keeping stock axles then 4.56 should be good to go all the way unless you are building a heavy overland rig or towing a camper.

 

[WTX Diesel]

What about 35" tite

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.

What about 3" and 35"?

 

[MCJA]

What about 35" tite
What about 3" and 35"?

For 35" tires, keeping the stock 3.73 gears results in a 6.24% loss in torque and same number gain in MPG. (Again, just talking about gearing numbers here. That doesn't take into consideration the added weight of the tires.)

If you use 4.10 gears with 35" tires, you gain 3.35% torque and lose the same amount in MPGs.

3" tires are really hard to find, unless you have the JLego conversion kit. You'll need to use portal axles because your tires have a smaller diameter than your ring gear, and your differential housing will contact the ground. You'll also need to modify your shock mounts, since they hang down below the axle. On the bright side, 3" tires tend to be solid rubber rather than inflatable, so you won't need an on-board air setup. You'll also eliminate any tire rub issues you may have - no need to trim your fenders!!

Ok, kidding aside... what other tire diameter did you mean?

 

[ogo]

For 35" tires, keeping the stock 3.73 gears results in a 6.24% loss in torque and same number gain in MPG. (Again, just talking about gearing numbers here. That doesn't take into consideration the added weight of the tires.)

If you use 4.10 gears with 35" tires, you gain 3.35% torque and lose the same amount in MPGs.

3" tires are really hard to find, unless you have the JLego conversion kit. You'll need to use portal axles because your tires have a smaller diameter than your ring gear, and your differential housing will contact the ground. You'll also need to modify your shock mounts, since they hang down below the axle. On the bright side, 3" tires tend to be solid rubber rather than inflatable, so you won't need an on-board air setup. You'll also eliminate any tire rub issues you may have - no need to trim your fenders!!

Ok, kidding aside... what other tire diameter did you mean?

Perhaps he was indicating a 3" lift?

 

[ytry]

I have stock trussed diffs with 37's and went to 4.56. I feel like with this configuration 4.11 would be the perfect all round gear ratio for the JLURD.

 

[MCJA]

I have stock trussed diffs with 37's and went to 4.56. I feel like with this configuration 4.11 would be the perfect all round gear ratio for the JLURD.

I think you're right. Decades of gas Jeeping tells us that lower gearing is better. But the EcoDiesel produces so much torque compared to the weight of the vehicle that you don't need to compensate with lower gearing.

All of the math stuff is a great place to start. But nothing beats experience.

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