Re-gear JT Rubi on 38/40's??

[Gladi8r_22]

Hello all,
So here is my setup I have
22' JT Rubicon DIESEL
Curently running a 38x13.5 from stock 33's
Still stock 44 Axle's, stock geared at 3.73.
On a flat highway, I can get it into 8th no prob. doing 71-74 mph. & anywhere between 1500-1900 RPM. Milage is floating around 19.5-20.3 (this part doesnt really matter to me anyway, just for info). As soon as I start hitting any kind of hill it wants to drop right back into 7th and if I push it at all sometimes even down to 6th.

Here is where I am stuck, everything I hear and read says "you should go no less than a 4.88 or even a 5.13 setup".

I'm worried if i go that low of gear its gonna drive my RPM's too high, obviously diesels don't like that. Research looks like about 4.30 is almost bringing me back to stock setup, but never heard of that being sold, so I figured a 4.56 would be a good compromise. Also, will that still be good if I decide to go to a 40 down the road??

I do wheel it, mild to moderately, nothing extreme, not a flog and bounce either.

I was hoping to get some fellow diesel users input and experience on this, but of course any input is welcome too.

Thank you

Sponsored

 

[Minty JL]

With a diesel, 4.10s or 4.56s should be more then enough because you have low end torque; unlike use with the 3.6 gasser

I'm going to 5.13s because in the future I will move from 37s to either 38s or 40s. Right now with almost 20k on the clock with 37x13.5s and OEM 4.10s it isn't horrible. I have been everyplace on the east coast from NC to ME

MPGs is not a concern for me though. I work remote and only on site a couple times a week and the commute is only 10 miles each direction. When I road trip my JTM, I have already built fuel cost into my trip

 

[Gladi8r_22]

With a diesel, 4.10s or 4.56s should be more then enough because you have low end torque; unlike use with the 3.6 gasser

I'm going to 5.13s because in the future I will move from 37s to either 38s or 40s. Right now with almost 20k on the clock with 37x13.5s and OEM 4.10s it isn't horrible. I have been everyplace on the east coast from NC to ME

MPGs is not a concern for me though. I work remote and only on site a couple times a week and the commute is only 10 miles each direction. When I road trip my JTM, I have already built fuel cost into my trip

Thanks for the input on this, I was wondering about even 4.10's but sometimes I hear folks say going to 4.10's is not worth the money on return. But then again, I'm not sure where their logic may lay, I'm not looking to do burnouts either. HAHA!!

Thanks

 

[DarthAWM]

 

[PuddleJumper]

4.10s if you gonna stay with that tire size. 4.56 if you plan on going bigger. 5.13 and 4.88 are only practical for gasser applications in most cases.

 

[CrazyCooter]

I run 4.10's on 38's and tow/load right up to max GCVWR ,run in the mountains mostly with 8-12% grades regularly, and wouldn't want it any other way.

I knew that 10% change on 37's was going to suck, but 38's were in the plans and would spit some of 10% the difference. I did one 3200 mile trip on the 37's with just a tent on the truck and camping gear......didn't like it on the interstate. Put the 38's on after that trip and she is all happy now!

I did this so that I would use all 8 gears when unloaded, 7 when towing, and I could run 6th (direct) when towing into the wind or uphill at heavy throttle position. I found with 3.73's on 37's that sometimes the truck would drop to 5th into a big headwind and I didn't like the idea of running an underdrive gear for long distance.

I run a gear shop in NorCal.....Yes, I generally advise people against spending the money on a gear job for less than a 10% change UNLESS it's a diesel and then 4-7% can make all the difference.

 

[Minty JL]

Thanks for the input on this, I was wondering about even 4.10's but sometimes I hear folks say going to 4.10's is not worth the money on return. But then again, I'm not sure where their logic may lay, I'm not looking to do burnouts either. HAHA!!

Thanks

Yeah anything beyond 4.56s with the 3.0 is overkill IMO..............unless you plan on going over to Ton axles and 42s

 

[ATX_Jeeper]

Teddy from Unwinding Roads just went from 37s to 38s on his JTRD with 4.56s and stated that the 38s lugged the engine and caused higher EGTs. He is also on the EOC stage 2 tune. Have a look. Video Here

 

[Vtur]

This question pops up quite often and no one will have the correct answer for your usage needs. But, here are some data for comparison with what you're currently running. I took these reading after a few seconds holding at those speeds.

Diesel 4.10s on 37s that has approx 25k miles, so the rpm might be slight higher due to worn tires.

 

[Gladi8r_22]

4.10s if you gonna stay with that tire size. 4.56 if you plan on going bigger. 5.13 and 4.88 are only practical for gasser applications in most cases.

If I go with 4.56 that still leaves me with enough room to go to a 40 and still have a gain?

Thanks

 

[PuddleJumper]

If I go with 4.56 that still leaves me with enough room to go to a 40 and still have a gain?

Thanks

Oh definitely. I got a 7.3 power stroke on 4.10s with 37s and tbh it’s too torquey. 4.56s on 40s w/ diesel would be a sweet spot for good highway but plenty for towing/offroad

 

[Gladi8r_22]

This question pops up quite often and no one will have the correct answer for your usage needs. But, here are some data for comparison with what you're currently running. I took these reading after a few seconds holding at those speeds.

Diesel 4.10s on 37s that has approx 25k miles, so the rpm might be slight higher due to worn tires.

Thank you for the input and the data to show. Very much appreciated.

 

[Gladi8r_22]

Oh definitely. I got a 7.3 power stroke on 4.10s with 37s and tbh it’s too torquey. 4.56s on 40s w/ diesel would be a sweet spot for good highway but plenty for towing/offroad

I think the 4.56 will be the way to go for me cause I am really considering go to 40's, I forgot to mention I do tote around the decked drawer system stuffed & at some point will want to add one of these too.

Thanks all for the help & input on this.

 

[Tennessee Gobi]

Saw this post the other day and just got a chance to get back to it. I've got a 22 JT Rubicon Diesel and I'm running 39" BFG KM3's re-geared to 4:56 and it's the perfect gear choice with the Diesel. Pulls and shifts just like it did with stock tires and the 3:73 gears and I still get 20+ MPG on the highway.

The BFG's run a little small, so they measure out around 38 1/2, which is perfect for my setup. With the Diesel, you could run 40's with the 4:56 gears no problem, but would need to keep the wheeling moderate running 40's on stock axles.

 

[Gladi8r_22]

Saw this post the other day and just got a chance to get back to it. I've got a 22 JT Rubicon Diesel and I'm running 39" BFG KM3's re-geared to 4:56 and it's the perfect gear choice with the Diesel. Pulls and shifts just like it did with stock tires and the 3:73 gears and I still get 20+ MPG on the highway.

The BFG's run a little small, so they measure out around 38 1/2, which is perfect for my setup. With the Diesel, you could run 40's with the 4:56 gears no problem, but would need to keep the wheeling moderate running 40's on stock axles.

This is great information, thank you. I agree, if I decide to go to the 40's it will be mostly moderate wheeling for sure.

Sponsored