Gearing and transfer case on diesel with larger tires

[rcr1340]

I am thinking about ordering a diesel. I'm just not sure which package. With all the torque the diesel has it doesn't seem to me a 4:1 transfer case is needed. I have owned Rubi's before and loved the 4:1 BUT they were gas powered. When most companies convert a Wrangler to a V8. they replace the 4:1 transfer case with a 2.72 transfer. They say it has more torque and doesn't need it. Too low. I do plan on rolling on 39s and usually mod my Wranglers pretty heavy. What have ya's found with the diesel when larger tires are added?

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[Motoristx]

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.

 

[40”JLURD]

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.

Nice build! I am about to take delivery of my Granite Crystal 2021 JLUR Diesel in a couple weeks and can’t wait!

I plan on running Yokohama X MT in the 40” flavor, and I’ve been debating between 4.56 and 4.88... from looking at the available 3.6L Pentastar gear ratio charts (can anyone find a diesel specific one yet?) it looks like 4.56 would keep it closest to factory.

However my builder who said he’s done a couple diesels already insists that 4.88 is better and Jeep “is under gearing the diesels a little from the factory”

Also another member on here (can’t remember who, damnit) has a diesel and put in 4.56’s and 37’s then afterward went to 40’s on the same gears. He said the 4.56 felt perfect with the 37’s and now that he’son the 40’s he wished he had gone with 4.88...

Your post above seems to agree with my first thought that the 4.56 would be perfect with 40’s though because of the lower powerband in the diesel.

Now I’m in a quandary... which to go with... what do you guys think???

 

[40”JLURD]

I am going to DD this beast too so better MPG will be welcome

 

[Motoristx]

I wouldn’t trust any opinions of anyone that doesn’t actually drive one. On the stock tires with 3.73, I think 4.10s would have been too much. This is based on my experience of driving it every day. The 3.73s just had a great balance on power/economy for a DD. If the 2021 has an option for 4.10s, I’d recommend getting the 4.10 with the intention of running 40’s. With 40’s a 4.56 would probably be good, but I’d personally want to try a 4.10 first.

 

[SolarWizard]

4.10 would match the RPM band with 37s compared with 3.73 to the stock 33s

 

[oceanblue2019]

Nice build! I am about to take delivery of my Granite Crystal 2021 JLUR Diesel in a couple weeks and can’t wait!

I plan on running Yokohama X MT in the 40” flavor, and I’ve been debating between 4.56 and 4.88... from looking at the available 3.6L Pentastar gear ratio charts (can anyone find a diesel specific one yet?) it looks like 4.56 would keep it closest to factory.

However my builder who said he’s done a couple diesels already insists that 4.88 is better and Jeep “is under gearing the diesels a little from the factory”

Also another member on here (can’t remember who, damnit) has a diesel and put in 4.56’s and 37’s then afterward went to 40’s on the same gears. He said the 4.56 felt perfect with the 37’s and now that he’son the 40’s he wished he had gone with 4.88...

Your post above seems to agree with my first thought that the 4.56 would be perfect with 40’s though because of the lower powerband in the diesel.

Now I’m in a quandary... which to go with... what do you guys think???

Interesting that he said they are under-gearing from factory.

I know that the guys I go out with that have 37's and re-geared both get much (upwards of 3mpg on the highway) better mileage than the JLUD with 35's and factory gears in our group.

 

[Robbyreneeward]

What about a non rubicon diesel? Mine has 3.73 rear axle ratio. Would you need to regear for 37s?

 

[Motoristx]

What about a non rubicon diesel? Mine has 3.73 rear axle ratio. Would you need to regear for 37s?

For the 2020’s all the EcoDiesels had the same axles and gears. Of course the non Rubicons didn’t get lockers or the 4:1 T Case, but other than that the drivetrain is the same. As for road driving, there wouldn’t be a difference and the stock 3.73 gears would be fine for 37’s, just like the Rubicons. You’ll be in 8th gear around 60 mph, and it will stay in 8th except for going up hills and on overpasses. I’d recommend trying the stock gears for a few thousand miles and then making a decision.

 

[oceanblue2019]

What about a non rubicon diesel? Mine has 3.73 rear axle ratio. Would you need to regear for 37s?

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.

 

[SolarWizard]

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

 

[oceanblue2019]

For the 2020’s all the EcoDiesels had the same axles and gears. Of course the non Rubicons didn’t get lockers or the 4:1 T Case, but other than that the drivetrain is the same. As for road driving, there wouldn’t be a difference and the stock 3.73 gears would be fine for 37’s. You’ll be in 8th gear around 60 mph, and it will stay in 8th except for going up hills and on overpasses. I’d recommend trying the stock gears for a few thousand miles and then making a decision.

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.

 

[oceanblue2019]

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

Please re-read his question, and my answer.

 

[Motoristx]

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.

I’d say something is off with that... that’s the experience I had when I tried 40’s on mine. I kept the calibration for the 38’s and 3.73’s. Just slapped the 40’s and the mileage was about 3 mph less than gps at 75 mph. Would only go to 8th when it was forced. Needless to say, the 40’s didn’t work with 3.73s, but the 38’s definitely work. So for 35’s to act like that would be cause for concern. Check out Casey 250 on YouTube, he’s running 37’s on stock 3.73s also.

 

[Robbyreneeward]

I’d say something is off with that... that’s the experience I had when I tried 40’s on mine. I kept the calibration for the 38’s and 3.73’s. Just slapped the 40’s and the mileage was about 3 mph less than gps at 75 mph. Would only go to 8th when it was forced. Needless to say, the 40’s didn’t work with 3.73s, but the 38’s definitely work. So for 35’s to act like that would be cause for concern. Check out Casey 250 on YouTube, he’s running 37’s on stock 3.73s also.

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.

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