Diesel JT Ready For New Gears ⚙️

[Tennessee Gobi]

I have a 22 JTDR and I’m running 39 X 13.50-17 BFB KM3’s which are closer to 38 inches than they are 39’s. I re-geared to 4.56 gears and it drives, shifts and accelerates basically the same as it did with stock gearing and tires.

If you were happy with the way it pulled stock, go with 4.56. If you want a little more on the low end, go with 4.88 gears. But if you go with 4.88’s you are going to need to move to the 38’s ASAP.

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[CreepyJeepy]

I am on 37" Toyo Open Country Trail R/T tires and have re-geared to 4.56. (3.0 eco)

I love the torque, and just the overall less strain on the motor. The mechanical advantage is real.

I would not do 4.88 on 37's It will be to much... I also have an S&B intake and find that I'm at lower boost PSI a lot of the time now, which makes the intake sounds amazing... High boost on 3.0 the turbo is nearly silent, but 4-10psi it sounds amazing.

I find about 2150-2250 RPM at 75mph right now, which is about perfect.... 4.88 on 37 would make 80 mph RPM likely too high and efficiency would suffer.

4.56 G2 gears here. Very happy with them....

 

[Stan H]

Nitro makes a 4.30 ratio for the Advantek axles.

https://eastcoastgearsupply.com/i-2...MqlTMK6cK3MrocIiA0RCBoUAeUTSfgSLJhnv5bH1ar5L4

How are those guy's they aren't out of my realm of travel for a regear

 

[VA6489]

Yes please. I would be interested to know why you are gong that way.

Bought mine (JTRD 2021) from a friend with the 4.10 axle swap and 37in tire. I run BFG KM3 39s now and swaped the gears for 4.56.

After a 5700 mile round trip to the Rubicon from VA this is what I have for hard data.

Fuel mileage over all 20.7 to include the rubicon trail.

Rig was fully loaded with a above cab RTT on an external rack.

Highway runs easily at 70,80,90MPH. Running hills fully loaded at speed OIL temps began to rise easily to 250 degrees. this required down shift and slowing down to control temps.

My rig is dealted.

The rig I went with is a 2023(?) JTRD 4.56s on SST Pro 40s.
Fuel mileage over the same duration 18.7

Rig included a Aluma Cap and hard RTT. More streamlined than mine.

Similar issues with Oil temp at speed and grades.
His rig is not deleted.

Driveability. We both love the 4.56 gears for round town driving. They work well paired to the 4:1 t case gears off road. Off road this allows more 2,3 gear running in low range in the rocks. Back to the highway, We understand this is not a 400 Cummins diesel w 1200 torques so pulling grades you need to watch the gauges or the motor will derate.

Discussions with axle companies Dana, Currie and East Coast Gear and supply with a BFG 39 I am on the edge between 4.56 and 4.88. If you plan to travel heavy or tow a trailer the 4.88 for the larger tire would be the way to go. If you just use you truck as a toy and daily 4.56 is not an issue.

Note:
Mileage with 3.73 to 4.10 improved 2 MPG with 37s From 4.10s to 4.56 milerage improved 2-3 mpg. Deleting the emissions mileage improved 2-3 MPG unloaded.

My current configuration 4.56 gears and BFG 39s 62-65 is the sweetspot for efficency.

Hope this helps

Cheers

 

[Wheelin98TJ]

How are those guy's they aren't out of my realm of travel for a regear

I've never dealt with them, but I've heard many good things. They've been around for quite awhile now. Probably 20 years.

 

[Stan H]

I've never dealt with them, but I've heard many good things. They've been around for quite awhile now. Probably 20 years.

Think I am gonna call em .

 

[Tulbox]

Greetings all. This is exactly the thread I have been hoping to find. I purchased a 2021 JT Diesel that was 28 months old with only 5600 miles. I did have problems the first year as right away it had a bad turbo which took some time to diagnose and once repaired all other problems were related to poor workmanship of the dealer. Turns out the warranty that mopar offers does not cover workmanship at another dealership.

Enough on that! I pull an RV (no Stan not a Class A or C, though I would not hesitate in pulling one out of a ditch if they were in distress) a 24' trailer with a empty weight of 4200 lbs, but I have had it close to 7K before I realized it. My rig pulled it well even pulling 6-7% grade doing 40-50 mph using manual shift with no problems (once the turbo was replaced). I intend on raising the ground clearance by adding a lift kit of about 3-3.5 in and going to 37" tires. I feel the stock 3.73 gears of the diesel package will be to cumbersome with pulling a 6-7K trailer unless I wished to stay in 5th or 6th gear.

I have checked numerous charts and computed using several different formulas I have found on the net and they all give similar findings. Going to 37" tires from 3.73 gears, If I could find 4.27 gears it would equal the same as stock. Going to 4.56 I would increase my given RPM by +- 250. If I go to 4.88 the RPM would increase by about 770.

There are several shops out there that specialize in re-gearing and I reached out to one located in Texas who responded by saying the best gearing for the diesel was 4.56 which was the same they would do for the v-8.

While I have not heard any comments from a diesel owner that tows a heavy trailer, from what I have read in this thread I would feel that the 250ish rpm gain using 4.56 over 3.73 would more than compensate for the larger diameter of the heavier tires.

When I do the gears, based on the information to date and that I do heavy towing I feel comfortable going to 4.56 gears.

 

[Stan H]

Greetings all. This is exactly the thread I have been hoping to find. I purchased a 2021 JT Diesel that was 28 months old with only 5600 miles. I did have problems the first year as right away it had a bad turbo which took some time to diagnose and once repaired all other problems were related to poor workmanship of the dealer. Turns out the warranty that mopar offers does not cover workmanship at another dealership.

Enough on that! I pull an RV (no Stan not a Class A or C, though I would not hesitate in pulling one out of a ditch if they were in distress) a 24' trailer with a empty weight of 4200 lbs, but I have had it close to 7K before I realized it. My rig pulled it well even pulling 6-7% grade doing 40-50 mph using manual shift with no problems (once the turbo was replaced). I intend on raising the ground clearance by adding a lift kit of about 3-3.5 in and going to 37" tires. I feel the stock 3.73 gears of the diesel package will be to cumbersome with pulling a 6-7K trailer unless I wished to stay in 5th or 6th gear.

I have checked numerous charts and computed using several different formulas I have found on the net and they all give similar findings. Going to 37" tires from 3.73 gears, If I could find 4.27 gears it would equal the same as stock. Going to 4.56 I would increase my given RPM by +- 250. If I go to 4.88 the RPM would increase by about 770.

There are several shops out there that specialize in re-gearing and I reached out to one located in Texas who responded by saying the best gearing for the diesel was 4.56 which was the same they would do for the v-8.

While I have not heard any comments from a diesel owner that tows a heavy trailer, from what I have read in this thread I would feel that the 250ish rpm gain using 4.56 over 3.73 would more than compensate for the larger diameter of the heavier tires.

When I do the gears, based on the information to date and that I do heavy towing I feel comfortable going to 4.56 gears.

The power will be there but be fore warned when you jack a vehicle suspension you loose tow and payload capacity . If your pulling a 24ft trailer your probably gonna run into difficulties and should consider load leveling bars .
4.56 is where most of the D owners are going.

 

[Jteakus]

I would like to add that after much research I have changed my plans and have purchased Dana 4.56 gears to install. My tires are 38 Nitto (37-5/8") and I am heavy with camp gear and roof top tent on full height bed rack.

 

[VA6489]

Greetings all. This is exactly the thread I have been hoping to find. I purchased a 2021 JT Diesel that was 28 months old with only 5600 miles. I did have problems the first year as right away it had a bad turbo which took some time to diagnose and once repaired all other problems were related to poor workmanship of the dealer. Turns out the warranty that mopar offers does not cover workmanship at another dealership.

Enough on that! I pull an RV (no Stan not a Class A or C, though I would not hesitate in pulling one out of a ditch if they were in distress) a 24' trailer with a empty weight of 4200 lbs, but I have had it close to 7K before I realized it. My rig pulled it well even pulling 6-7% grade doing 40-50 mph using manual shift with no problems (once the turbo was replaced). I intend on raising the ground clearance by adding a lift kit of about 3-3.5 in and going to 37" tires. I feel the stock 3.73 gears of the diesel package will be to cumbersome with pulling a 6-7K trailer unless I wished to stay in 5th or 6th gear.

I have checked numerous charts and computed using several different formulas I have found on the net and they all give similar findings. Going to 37" tires from 3.73 gears, If I could find 4.27 gears it would equal the same as stock. Going to 4.56 I would increase my given RPM by +- 250. If I go to 4.88 the RPM would increase by about 770.

There are several shops out there that specialize in re-gearing and I reached out to one located in Texas who responded by saying the best gearing for the diesel was 4.56 which was the same they would do for the v-8.

While I have not heard any comments from a diesel owner that tows a heavy trailer, from what I have read in this thread I would feel that the 250ish rpm gain using 4.56 over 3.73 would more than compensate for the larger diameter of the heavier tires.

When I do the gears, based on the information to date and that I do heavy towing I feel comfortable going to 4.56 gears.

That is a lot of weight, Gearing to 4.56 is the right thing to do with a 37inch tire. The next thing you will have to address is Oil temp blowing thru 250 degrees. The oil cooler is really small while external it is not in the direct airflow. Couple that to the fact it is cooled by engine coolant Things can get quite hot.

The simple solution is to slow down on a grade and down shift . yes this puts you right back where you started and the reason for the gear change. But it doesn't address the problem of heat. JTs have a small radiator and a packed engine compartment. Air flow is an issue.

Just some things to add to you calculations.

 

[Zachanadandy]

The power will be there but be fore warned when you jack a vehicle suspension you loose tow and payload capacity . If your pulling a 24ft trailer your probably gonna run into difficulties and should consider load leveling bars .
4.56 is where most of the D owners are going.

I know everyone says this, because logically being taller and less aerodynamic with larger tires would reduce those things...if they were the limiting factor in towing and payload. What if they aren't at all? What if the limiting factor for payload is in fact the very soft factory springs? Any aftermarket lift spring would actually increase the payload capacity as they all have higher rates than stock. What if the towing capacity is really limited by the cooling system and even there mostly because of the stupid Davis dam test that is required for towing certification and must be run when it's 100⁰ outside? If you don't tow up steep grades in the desert heat, I'd argue the lift and tires would have 0 effect on your tow capacity. All that being said, our Jeeps don't even get a GCWR rating on the door placard like most trucks so there's both no way to really calculate the difference from added weight or for a cop to try and enforce it anyway. The Jeep site is so terrible odds are even if you were hauling commercial and hitting the scales they'd only bat an eye if you were exceeding the max tow rating of 7700 pounds. The rest of us are never getting weighed. Nobody cares if you're still towing 7k with your lifted truck rated at 6k to start. We can't change the ratings either direction. Lowering the truck, putting smaller tires, heavier springs, and improving the cooling system would logically make it tow better and handle payload better... but doesn't change the ratings. Lifting it and putting bigger tires on it will obviously make it handle those tasks worse... but again doesn't charge the ratings. If there's one thing I've learned building our cabin it's that the JT is capable of hard use. From an 8400 pound trailer in my lifted mojave on 37s towed 600 miles including large grades without issue (the largest gains 4k feet in 20 miles but I ran it at night in the winter so heat wasn't an issue) to 1800 pounds of pavers in the bed 50 miles including the 20 miles of washboard dirt road. The metalcloak 3.5" springs haven't batted an eye.

 

[Tulbox]

Yes, but keep in mind the only reason the Eco max tow capability is lower than the gas is due to cooling capacity, there just was not enough room to stuff in a bigger radiator for more cooling. While most of us are smart enough to downshift and climb the hill slower to reduce cooling temps many are not. They ended up designing all the limitations based on those who are not.

There are not any official statements regarding gross towing capacity. Case in point a ford ranger with a 4 cyl and class IV hitch is rated by ford for 7K. With the JT it goes like this. Just a bumper hitch capacity is 2K, add a class IV receiver and it is 4500, add the 8sp auto trans and it is 6K add 4.10 gears and it is over 7K. Interesting how 4.10s are standard on most of the JTs except if you have a diesel it is swapped for 3.73.

SAE does have a standard for tow rating, but who actually uses it. The real deciding factor on tow ratings is simply one manufacturer wants their truck to be equal or better that the competition in that class of truck. As far as legalities go I can not speak of every state, but having driven commercial trucks and then with 28 years in law enforcement I can say that LE only cares if a vehicle is over weight per axle or by licensed gross weight. If you have paid your fees for weight a commercial truck is good unless the weight is overloaded on one axle. Although if one was pulling say a 10K lb trailer they should be able to show that the brakes and equipment make it safe to do so.

It is my belief that the JT eco has a lower tow rating than the gas is do to the gas having room for a larger cooling system, so with all you back yard engineers out there lets come up with an add on radiator to enhance that cooling capacity! I mean like say a 4" high 4-5' wide radiator mounted above the windshield instead of a lightbar that is not needed. Just a thought.

 

[Zachanadandy]

Yes, but keep in mind the only reason the Eco max tow capability is lower than the gas is due to cooling capacity, there just was not enough room to stuff in a bigger radiator for more cooling. While most of us are smart enough to downshift and climb the hill slower to reduce cooling temps many are not. They ended up designing all the limitations based on those who are not.

There are not any official statements regarding gross towing capacity. Case in point a ford ranger with a 4 cyl and class IV hitch is rated by ford for 7K. With the JT it goes like this. Just a bumper hitch capacity is 2K, add a class IV receiver and it is 4500, add the 8sp auto trans and it is 6K add 4.10 gears and it is over 7K. Interesting how 4.10s are standard on most of the JTs except if you have a diesel it is swapped for 3.73.

SAE does have a standard for tow rating, but who actually uses it. The real deciding factor on tow ratings is simply one manufacturer wants their truck to be equal or better that the competition in that class of truck. As far as legalities go I can not speak of every state, but having driven commercial trucks and then with 28 years in law enforcement I can say that LE only cares if a vehicle is over weight per axle or by licensed gross weight. If you have paid your fees for weight a commercial truck is good unless the weight is overloaded on one axle. Although if one was pulling say a 10K lb trailer they should be able to show that the brakes and equipment make it safe to do so.

It is my belief that the JT eco has a lower tow rating than the gas is do to the gas having room for a larger cooling system, so with all you back yard engineers out there lets come up with an add on radiator to enhance that cooling capacity! I mean like say a 4" high 4-5' wide radiator mounted above the windshield instead of a lightbar that is not needed. Just a thought.

There is an aftermarket add on radiator for the EcoD that mounts under the front bumper.

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