Gearing for 37s to 40s??

[TheSolarWizard]

direct quote from adam at rockkrawler

”we did 4.10s with our shop JLD with 38s. We feel 4.56 would have been better”

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[JC JT]

direct quote from adam at rockkrawler

”we did 4.10s with our shop JLD with 38s. We feel 4.56 would have been better”

You do have diesel correct

 

[enjin]

I'm having a hard time keeping in upper 20's with factory tires and gears when loaded down with winch and camping gear. More like below 20. Need to regear probably, but fuel mileage falls off fast.

 

[Pizziola29]

JTRD...Low 20s mpgs at fwy speed on 40s with 4:88 gears. Will swap axles in next year. Used tazer for adjustments.

 

[BEERviper]

What's interesting is that Jeep chose to use the same 3.73:1 gears for all trim levels. The Sport S I have on order will come with 31.5" tall tires whereas the Rubicon comes with 33" tall tires.

The general rule of thumb that I have read about to keep the same/similar RPM at speed when going to larger tires is:

(New tire size / Old tire size) * Current Gear Ratio = New Gear Ratio

So for my Sport S:
(35/31.5) * 3.73 = 4.14 (round down to 4.10)
(37/31.5) * 3.73 = 4.38 (round up to 4.56)

For the Rubicon it's:
(35/33) * 3.73 = 3.95 (negligible difference)
(37/33) * 3.73 = 4.18

Why Jeep chose to keep the same gear ratio for all diesel trims is a bit of a mystery when they have different tire sizes.

If it comes down to fuel economy, it comes down to how much will it really cost to swap gears vs. how much improvement will you see? I don't have a shop nor easy/cheap/free access to 4.10 gears so I'm looking at probably $1,500. As of this writing that same $1,500 buys 842 gallons of diesel fuel!

Now when it comes to towing, I think that's a bit of a different story since a bigger gear will help to keep the revs in check when climbing. However I feel like the 8 speed auto combined with the 70% increase in torque over the Pentastar will make that less of an issue.

I will be interested to see your results after the regear OP!

 

[ssteve]

Anyone who says you shouldn't or your better off not re-geaing isn't at all concerned about loading the truck or performance and are only talking in theory or based off fuel mileage concerns. even with just 37's there's major benefits of lower gears. Less strain on the trans, multiplied tq etc. Manufactures gear vehicles to meet epa and other regulatory standards, not for performance and that's on level ground in ideal situations. Not at elevation, with hot weather up a massive grade, with a huge payload and 100lbs of rotational weight added on each corner. Watch some of Casey250's videos. He had his jeep throw a code on him cause he was wot going up hill and his jeep was losing speed. If he was on 3.21 gears or whatever he wouldn't be able to pull that hill in anything but 1st and 2nd gear.

Ill be running 38" tires and plan to go to about a 4.55 gear. Anything lose in mpg on the freeway it will be made up in fuel mileage in town and on the trail and I wont have to be concerned about trans temps etc. Jeep has already said the limiting factor in the ecodiesel in the jeep is heat management, not being geared low enough to handle additional load of larger tires isn't going to help that at all.

 

[TheSolarWizard]

JTRD...Low 20s mpgs at fwy speed on 40s with 4:88 gears. Will swap axles in next year. Used tazer for adjustments.

2000 @ 60 in 8th seems a little high. When your break-in is over Would you mind reporting back what RPM is at 70, 75 and 80?

 

[JTRdiesel]

Anyone who says you shouldn't or your better off not re-geaing isn't at all concerned about loading the truck or performance and are only talking in theory or based off fuel mileage concerns. even with just 37's there's major benefits of lower gears. Less strain on the trans, multiplied tq etc. Manufactures gear vehicles to meet epa and other regulatory standards, not for performance and that's on level ground in ideal situations. Not at elevation, with hot weather up a massive grade, with a huge payload and 100lbs of rotational weight added on each corner. Watch some of Casey250's videos. He had his jeep throw a code on him cause he was wot going up hill and his jeep was losing speed. If he was on 3.21 gears or whatever he wouldn't be able to pull that hill in anything but 1st and 2nd gear.

Ill be running 38" tires and plan to go to about a 4.55 gear. Anything lose in mpg on the freeway it will be made up in fuel mileage in town and on the trail and I wont have to be concerned about trans temps etc. Jeep has already said the limiting factor in the ecodiesel in the jeep is heat management, not being geared low enough to handle additional load of larger tires isn't going to help that at all.

4.56 puts you at about 1750rpm at 65mph on 38s that should be reasonable even on the highway, the more I look at it I wanna do 4.30s. To keep RPms in check during the winter on 35” Nokian Tires, (maybe 37” K02s?)

 

[BEERviper]

4.56 puts you at about 1750rpm at 65mph on 38s that should be reasonable even on the highway, the more I look at it I wanna do 4.30s. To keep RPms in check during the winter on 35” Nokian Tires, (maybe 37” K02s?)

I wasn't aware there was a 4.30 gear size for the Dana 44? I thought it was 4.10 then 4.56 then 4.88?
I'm running 4.88 in my 2016 JKU which for me is actually overkill for any highway driving. It's great off the line but it revs through 5 gears rather quickly and also doesn't shift when it should. I don't think that will be an issue with the 8 speed though.

 

[JTRdiesel]

I wasn't aware there was a 4.30 gear size for the Dana 44? I thought it was 4.10 then 4.56 then 4.88?
I'm running 4.88 in my 2016 JKU which for me is actually overkill for any highway driving. It's great off the line but it revs through 5 gears rather quickly and also doesn't shift when it should. I don't think that will be an issue with the 8 speed though.

I should mention I’m doing XD60s or Possibly Currie F9s if I can get them to Canada, but nitro is producing a 4.30 Gearset for the JL/JT Axles not sure if they are for sale yet.
https://www.nitro-gear.com/Nitro-Dana-44-220mm-4-30-p/m220jl-430-ng.htm

 

[BEERviper]

I should mention I’m doing XD60s or Possibly Currie F9s if I can get them to Canada, but nitro is producing a 4.30 Gearset for the JL/JT Axles not sure if they are for sale yet.
https://www.nitro-gear.com/Nitro-Dana-44-220mm-4-30-p/m220jl-430-ng.htm

Good to know, thanks for the link! That could be a great medium between the 4.10 and 4.56!

 

[ssteve]

I should mention I’m doing XD60s or Possibly Currie F9s if I can get them to Canada, but nitro is producing a 4.30 Gearset for the JL/JT Axles not sure if they are for sale yet.
https://www.nitro-gear.com/Nitro-Dana-44-220mm-4-30-p/m220jl-430-ng.htm

That's awesome news and a perfect ratio for these trucks imo. I have a few weeks till mine gets here but I think that's what I will go with.

 

[Schreconjeep]

Well this is a very helpful thread. OP, I can't wait for a report when you've swapped. My JTRD with 37x13.50 STT pros does great with stock gears. It did lose3-5 mpg with the tires as expected. However, i took a really short trip with the new camper (23' 3700lbs) last weekend and it seemed to downshift a lot. Again it was a short trip ~25miles, but had me thinking about regearing again. 4.30s sound perfect but still not sure.

 

[houseofdiesel]

Hello All,
I have been in the diesel world before Diesels where even cool.
With that said i was a avid Cummins guy just sold my completely restored 1997 12V 4x4 to buy a new Rubicon Diesel.

My order just came in WHite Rubicon Loaded with everything you could get.
Took it home pulled all the stock suspension out and replace it with a 3.5 TeraFlex IR Full system.
Went with Nitto Terra Grapplers 37/12.50/17s
Here is my Question:
I want to regear to bring it back to stock
From what I know 373 gear needs to come up to a 410 gear to offset the 285/70/17 difference of the 37/12.50/17.
I want to know if anyone has regeared a Diesel JT or JL yet.

Before anyone says you don't need to please don't. I know to get it back in its TQ curve it needs more gear.
Thank you for your help
V/r JC

Not really a need to gear a diesel, especially with a Rubicon. The transmission will be the weak point but with the advances in clutches and converters you will actually enjoy better fuel economy on the freeway with larger tires. In the old days when we only had 3-5 speeds the gaps were too large in between gears and smoked transmissions. I've been hard core off roading and hauling for over 30 years now and never had a problem with a taller ratio in my diesels. The older stuff like my '96 Powerstroke I had to go to the ZF to get the strength and ratios to only run 4.88's on 42"s, but since Ford went to the 5R110 automatic I've never had a problem. We are running 35"s on my daughter's Canyon LWN 2.8L diesel and it never misses a beat and I've even towed a diesel 1980 Scout Traveler up through the Sierra Nevada mountains to Carson City and back down to the bay are with it zero issues. Hauled a 35' Gooseneck all the way to Winnipeg Canada to pick up a '57 Dodge Power Wagon and back with that '96 doing 80 mph and never had an issue on 42"s. You'll never notice the power loss with 37"s. With 40"s or 42"s I might go up one ration to 4.10 and that would be it unless it's a dedicated off road rig. My next diesel drag truck will be running 3.30 gears and I'm installing 3.73 in my F-450 here very soon.

 

[JC JT]

Well it's time for an Update, All is doing really well the 410s seem to be a good swap.
I wanted to use all my gears, again that is why I wanted do the swap.
I agree do you have to regear, NO is it best for the Jeep to add larger tires and not regear??
This is why I did what I did
LOL I have read through all these posting I don't criticise others for stepping outside the box and trying something new.
I have been building Diesels for years, I think I know what I am doing.
Thought we could be OPEN Minded here and post back results.
If it wasn't for people trying different things, where would we be now, Food for thought
If anyone wants to know more please ask.
I know a guy that went 456s he is happy taching 2K at 70mph,myself I thinks that's to much RPM on a diesel.
Getting ready to run down/over to Fort Bragg to se my Son, I will post some 70MPH average MPG
as of right now 60MPH is netting mid to lower 20 MPG. and we have nothing flat here so time will tell.

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