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Gearing and transfer case on diesel with larger tires

Motoristx

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I’m just really torn as to whether to run with 35s or 37s on my sport diesel. Thinking 3.5” lift (or more if need be), but I’d rather get what i really want from the jump vs settle on 35s and regret it. And by the way I do no off reading. Strictly a mall crawler type toy ride.
I knew before I got the Jeep (March 2020) that I wanted 38’s. So I specifically built it for 38’s. Didn’t want to do 35’s and step up, and have to readjust everything again. I’d say, if you are think of doing 37’s, do it. You can always put smaller tires on a bigger lift, can’t always put bigger tires on a smaller lift.
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kosinar

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I got 37 Cooper STT pro on KMC 235 beadlocks on my diesel Rubicon and will not even consider changing my gearing. That diesel have so much power I have to watch not to hit gas pedal too hard on wet road since my rear wheels will spin. Transmission is in 8th gear most of the time on the hwy except when accelerating. What would I be gaining? 40s or other heavy wheel/tire combination could be better with different gears but my setup is just about perfect. Like it was mentioned in previous post, do not take advice from someone not actually driving diesel with tire wheel combination similar to your desired setup.
 

40”JLURD

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I just spent 4 hours on the highway this weekend, JLUD Rubi with 35's and factory gears. It has been re-programmed for tire size. At 75mph it was in 7th gear most of the time; and would only find 8th when no wind and flat or downhills.

At the same time my 2.0L Rubicon was at 75mph in 8th with 4.11's. I'm on exact same 35's as the above.

We also had a JLUD Rubi on 37's with 4.56's with us for about 1/2 the drive and he was in 8th and reporting 3.2 mpg better fuel consumption than the 35 tired Diesel on factory gears.

So it's not just about torque output as both Diesels have my little 2.0L beat by a good margin; so something else is going on in the programming.

We even swapped who led the caravan, and who drove what to rule that out.
Thank you for this very useful addition to this thread.

It seems to help support three things.

1. 37/38” tires and 4.56 gears are a perfect combination for the new Diesel. I’ve heard 3 or 4 members on here say that combination is perfect.

2. 40” tires will probably be best with 4.88 gears (as suggested by my builder for 40”, and by another member on here who had 37’s and 4.56’s and said it felt perfect. Then he put 40’s on the same gears and said he regretted not going with 4.88 now that he had 40’s)

3. The theory that my builder told me “the diesel’s are slightly under geared from the factory” seems to be true. Both the 37/38” and 4.56 & the 40” and 4.88 combo are one R&P set deeper than factory would be.

Technically the 4.56 gear would be a perfect match for the 40’s to match the stock RPM/Tire Size but everyone loves the 37/38” tires on 4.56 gears.

Therefore based on the above observations it would appear as though the ideal R&P to tire size ratios for the DIESEL are as follows:

35” = 4.11

37/38” = 4.56

40” = 4.88

Going deeper than this may not and probably will not be beneficial for on road driving as the diesel motor powerband is so much lower than that of the gas models the lower RPM gives you more power contrary to how in a gas motor the slightly higher RPM would bring you closer to the powerband.

This chart is comparing 4.56 to 4.88 vs stock with 40” tires.

Jeep Wrangler JL Gearing and transfer case on diesel with larger tires 87090D73-D5D2-4A1A-99C4-0FF037BCF821
 
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40”JLURD

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For anyone interested 4.11 is factory match for 37” on the Diesel, 4.56 is 1 set deeper and what everyone is loving on these

Jeep Wrangler JL Gearing and transfer case on diesel with larger tires 28D570DB-5565-404C-A414-090DB78B4DE7
 

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I guess it totally depends on what you're looking for.

Do you want to stay in 8th gear constantly on the highway? Then gear the hell out of it.

Do you want maximum fuel efficiency? Stay 3.73 (or regear to 3.55 which would be even better).

It seems that there are different needs for different folks. I'm running 34x12.00s on my Ram with the same powertrain and 3.92 gears. It's overkill. 1st gear is useless. The truck stays in 8th gear on the highway EVEN WHEN towing my enclosed trailer. I see less MPG than guys with the same truck who are running 3.21 gears. The 3.21s are the sweet spot for the new diesel. You get better use of the low gears, but highway RPMs still stay where they should for a diesel. That's why I'll never buy the idea that guys are seeing 3 MPG gains on 4.56 gears.

Now, I could go on up to 37s and the 3.92 gears would probably be perfect. But after living with this engine and 8HP75 trans for 7 months I simply cannot imagine putting 4.56 gears in it. That's just completely nuts with an engine that has such a small power band and makes peak torque at 1,600 RPM.
 

Compression-Ignition

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It depends on how you drive, how you expect your Jeep to drive, etc.

I know guys with 37's and they swear it "drives fine" with factory gears.

I also know guys who want to re-gear with 35's who have 4.11's (I'm one from this camp).

What I do know is in almost all cases where gears are changed to match RPM to tire size mileage improves, in some cases drastically.

What is interesting is the Diesels seem to pickup a bit more then expected from the re-gear, which is why I found the comment above that they come a bit under-geared from the factory interesting as it would agree with what my circle of trail buddies who have JLUD's have found.
you’re comparing an ice engine experience to A diesel.

he’s asking about low range and the superior auto in the diesel wont care at all what low range you have
@SolarWizard
A diesel is still an internal combustion engine.

I think you might have meant to compare a spark ignition engine to a compression ignition engine?
 

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Rcr1340 being you are looking at 39’s, and assuming you are going to keep the stock axles, I’d recommend trusting both of them, installing chromoly or RCV shafts and 4.88 gears. Mostly because you mentioned you are going to load it down with mods.

Had said that, I’d also strongly encourage you to pass on modifying the factory axles and use that money to put, at a minimum, Dana 60’s under your rig. They will handle the tires you want to run much better than the factory axles. Likewise have them geared with 4.88’s. And the other reason I say go with 4.88’s is if you decide to go to 40’s in the future. Then you will have adequate gearing to do so.

Now if you plan on going bigger than 40’s in the future, then go with a 60/80 or a 60/70 axle combo and 5.13. My rig is going to be running 60/80’s and 43’s, which we feel 5.13’s will work good with 42’s-44’s on the diesel rigs. It should be done before Christmas and I’ll finally be able to find out if our math is correct.
 

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I’m on a 2020 EcoDiesel Rubicon. 3.5 in lift and 38 Milestar Pat’s. The stock 3.73’s are perfect for 38’s, and the 4-1 transfer case is great on trails, both with rocks and without rock crawling. I’ve been to Alpine loop and Moab with it, as well as Holy Cross trail. The trans is in 8th right ~60 mph, and I get ~21 mpg combo. Driving at 85+ on the highway I get closer to 18.5 mpg, so I like to keep it around 65 when I can.

We tried 40’s on this setup, and it doesn’t work with the stock 3.73’s. It’s just a bit too much, and the trans stays in 7th on the highway. I could force it to 8th in manual mode, but once it was back in drive it go back to 7th shorty. 40’s will need a regear to 4.10 at least and 4.56 would probably be best.

With that being said. 39’s would be a good question on if the trans would like them or not. I’m willing to say, 38 are just about the max one could do without regearing, and if 39’s worked they’d probably see 65mph before ever seeing 8th gear... but I’m skeptical if that would happen.

As for physically fitting 38 versus 40, 38’s fit the architecture of the wheel well so much better. There is rubbing in the rear on the plastic inner fender liners, but it’s not bad, will need 3 inches of bump stops in the rear, unless you willing to start cutting/removing stuff. 40’s don’t fit as well and cutting is required. 39’s will probably need cutting as well. In the front, the 38’s don’t rub, depending on backspacing, nothing will rub or need to be cut. 39’s will probably be close to the rock rail and rub the liner . 40’s need to cut the front of the rock rail, and will rub the liner getting close to half stuff. Of course, this is all with stock fenders.

Jeep Wrangler JL Gearing and transfer case on diesel with larger tires 28D570DB-5565-404C-A414-090DB78B4DE7

Jeep Wrangler JL Gearing and transfer case on diesel with larger tires 28D570DB-5565-404C-A414-090DB78B4DE7

Jeep Wrangler JL Gearing and transfer case on diesel with larger tires 28D570DB-5565-404C-A414-090DB78B4DE7
Nice setup! Looks great! want to do something very similar with my diesel what 3.5" lift did you use. Want to add a winch so I'm not sure what lift (heavy coils) they have to accommodate the extra weight in the front.
 

Motoristx

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Nice setup! Looks great! want to do something very similar with my diesel what 3.5" lift did you use. Want to add a winch so I'm not sure what lift (heavy coils) they have to accommodate the extra weight in the front.
Im using Rock Krawler springs front and rear. The front are 3.5 inch, but I just swapped out the rear for 4.5 inch. With 3.5 inch in both front and rear, I ended up with 1/2 lower in the rear.

Still running the stock 3.73 gears. I haven’t quite changed my tune about never swapping gears, but... I may be planning on using an off road trailer in the future.

I had to Flat tow the YJ the other day @3060 lbs. It didn’t want to shift into 8th. If this happens with an off road trailer, I’ll end up regearing. Sure it won’t weigh as much, but I’m thinking wind resistance might play a part. Will have to wait and see.
 

40”JLURD

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I guess it totally depends on what you're looking for.

Do you want to stay in 8th gear constantly on the highway? Then gear the hell out of it.

Do you want maximum fuel efficiency? Stay 3.73 (or regear to 3.55 which would be even better).

It seems that there are different needs for different folks. I'm running 34x12.00s on my Ram with the same powertrain and 3.92 gears. It's overkill. 1st gear is useless. The truck stays in 8th gear on the highway EVEN WHEN towing my enclosed trailer. I see less MPG than guys with the same truck who are running 3.21 gears. The 3.21s are the sweet spot for the new diesel. You get better use of the low gears, but highway RPMs still stay where they should for a diesel. That's why I'll never buy the idea that guys are seeing 3 MPG gains on 4.56 gears.

Now, I could go on up to 37s and the 3.92 gears would probably be perfect. But after living with this engine and 8HP75 trans for 7 months I simply cannot imagine putting 4.56 gears in it. That's just completely nuts with an engine that has such a small power band and makes peak torque at 1,600 RPM.
All this ASSUMES the powerband is at its peak in the 1,400 to 1,600 range. Well the factory probably wants you to believe that but I assure the Dyno says much different.

Peak TQ is reached in the 2,000 rpm range. Right where you end up with a 4.56 re gear.

Here is how it breaks down if you play the video in slowmo.

@2,097 RPM I hit 510 Ft Lb of TQ
It only goes up from there until
@2,829 RPM it hit its MAX TQ of 547 Ft Lb
TQ still stays above 500 Ft Lb until
@3,174 RPM it finally falls to 499 Ft Lb

Don’t get so caught up in the factory lying ass specs. Here is my actual diesel on a Dyno in reality. Stock for now. And

Yes it pumped out 547 foot pounds of torque at the rear wheels!!! Not the crankshaft. This is on a Mustang Dyno which is known to have about a 20% drivetrain loss on AWD / 4WD vehicles as compared to the power at the crank.

 
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drick

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I’m on a 2020 EcoDiesel Rubicon. 3.5 in lift and 38 Milestar Pat’s. The stock 3.73’s are perfect for 38’s, and the 4-1 transfer case is great on trails, both with rocks and without rock crawling. I’ve been to Alpine loop and Moab with it, as well as Holy Cross trail. The trans is in 8th right ~60 mph, and I get ~21 mpg combo. Driving at 85+ on the highway I get closer to 18.5 mpg, so I like to keep it around 65 when I can.
Thank you for this post & information, can i ask you one clarifying question which i think i know the answer to? Context - I have a similar setup (2021 JLURD, 3.5" lift, 37's with BFG MTA KM3's) and am just now starting to get a feel for the gearing / tire setup.

I assume your advice would be to run this setup for a while before considering re-gearing, correct? If so, and i decided to move to 4.56's which seems to be the ideal ratio per another string here - would you recommend moving up to larger tire (38 or 39) at the same time, or staying at 37's? The rest of the setup will be beefed up before this happens (truss, RCV's, etc) so it think the stock D44's should be able to handle this - at least for a while.

TIA
D
 

Jwobes

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I’m just really torn as to whether to run with 35s or 37s on my sport diesel. Thinking 3.5” lift (or more if need be), but I’d rather get what i really want from the jump vs settle on 35s and regret it. And by the way I do no off reading. Strictly a mall crawler type toy ride.
Then you’d be fine on either considering the diesel sport has the same axels/gear ratio minus the lockers as the rubicon. 3.5 lift is more than enough to fit either 35s or 37s. The man is running 38s with no issues and wheels it!

Piece of advise: get what you want the first time or you will end up spending double the money because you’ll end up getting it anyways.

Also, take the JL off road some time, you’re keeping the beast in a cage 😭
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