Gearing for 37s to 40s??

[JC JT]

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

Looks really good Buddy, My concern is the RPM's be at 2K I drive mine to a lot of Military locations so I want a better gear. Think you could have went 456 and been a lot closer
But I don't know what you do with your Rig, LOL I bet that thing will SNAP you Neck!! on take off LOL

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[Pizziola29]

Looks really good Buddy, My concern is the RPM's be at 2K I drive mine to a lot of Military locations so I want a better gear. Think you could have went 456 and been a lot closer
But I don't know what you do with your Rig, LOL I bet that thing will SNAP you Neck!! on take off LOL

It does lol. But for now it works for what I need. I’ll update as I use it more.

 

[JC JT]

 

[Saltymedic]

Sorry to differ in opinions here but go with 4.56. Fuel milage will be shot no matter what you go with so go with the gear set best matched with 37's and the tranny.

 

[jurfie]

Ok, I just ran the numbers on Grimm Jeeper's gear ratio site.

4.10s with 37s (assuming 36.5" actual size) will be almost exactly the same as 3.73s with stock Rubicon tires (285/70R17).

Jumping to 40s (assuming 39.5" actual size), 4.56s would be best with the diesel.

This is assuming y'all are happy with the shift points when stock.

 

[BEERviper]

Ok, I just ran the numbers on Grimm Jeeper's gear ratio site.

4.10s with 37s (assuming 36.5" actual size) will be almost exactly the same as 3.73s with stock Rubicon tires (285/70R17).

Jumping to 40s (assuming 39.5" actual size), 4.56s would be best with the diesel.

This is assuming y'all are happy with the shift points when stock.

After I thought more about this and calculated the new gear ratio to be as close to the same RPM at Speed as stock, this does not take into account the added unsprung weight and it's effect on drivability.

Also such a big difference with the 70% increase in torque from ecoDiesel that the taller gear may really only be needed if towing often. A few of the towing videos I've seen this engine will still hold 8th gear towing a trailer on oversize tires due to the torque.

I wish TFL would hurry up and post their diesel Gladiator towing the Ike Gauntlet already so it can put that ridiculous MotorTrend article to rest saying that it couldn't maintain speed going up a hill while towing

 

[jurfie]

After I thought more about this and calculated the new gear ratio to be as close to the same RPM at Speed as stock, this does not take into account the added unsprung weight and it's effect on drivability.

Also such a big difference with the 70% increase in torque from ecoDiesel that the taller gear may really only be needed if towing often. A few of the towing videos I've seen this engine will still hold 8th gear towing a trailer on oversize tires due to the torque.

I wish TFL would hurry up and post their diesel Gladiator towing the Ike Gauntlet already so it can put that ridiculous MotorTrend article to rest saying that it couldn't maintain speed going up a hill while towing

Agreed; I’ve yet to drive a diesel, which is why I prefaced with assuming the shift points when stock are good. I’d probably drive it stock for a while (ugh) then see how it performs after adding bigger tires. The 3.6 is most efficient around 2000 RPM, but I’m not sure where the sweet spot is with the 3.0. Given the 8HP75 and the 8RE50 are geared the same, I’d guess it prefers cruising at a lower RPM since the 3.0 Rubicon gets 3.73s instead of 4.10s like the 3.6.

 

[JC JT]

Ok, I just ran the numbers on Grimm Jeeper's gear ratio site.

4.10s with 37s (assuming 36.5" actual size) will be almost exactly the same as 3.73s with stock Rubicon tires (285/70R17).

Jumping to 40s (assuming 39.5" actual size), 4.56s would be best with the diesel.

This is assuming y'all are happy with the shift points when stock.

That's what I have been trying to tell people LOL thank you for confirming me

 

[Saltymedic]

That's what I have been trying to tell people LOL thank you for confirming me

The gear ratio numbers align, yes. But have you considered also the added overall weight, rotational mass, road resistance, wind resistance?

4.10's may be ideal in a single numerical world but maybe the next step up would cover the add-ons?!

 

[houseofdiesel]

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.

200k miles 3.31 gear on 37" tires and it does ranch duty often. Just got back from a round trip from Cali to Texas and back. Mobbed through 2' of snow pack to go around all the traffic and dodge all the semi stuck in the middle of the road. These newer transmissions do not need lower gears unless max towing, overlanding, or hard core off roading. With how the multiple gear sets mesh together it shares the load through 3 clutches and 2 brakes per as opposed to the older transmissions taking all the load on just two gears. During one leg of the trip I ran at 70-75 mph and was able to run 712 miles on 42 gallons of fuel for just under 16.9 miles per gallon.

I had a manual cummins truck that ran the quarter mile faster on 40"s with 3.42 gears than it could with 33" drag radials due to having to shift less. Lowering gears only is making your trans shift harder and gaining back a little bit of the horsepower being delivered to the ground. Maybe a 5% gain back in HP max. I did dyno figures before on my older 5 speed auto and the losses of that power were pretty significant at about 25 hp per inch uncorrected and that was on a 500 rwhp comp truck. With the newer 6, 8, and 10 speed transmissions those losses are much smaller.

Now on my gas powered 4.6 SC with a C4 auto Bronco on 42" tires the losses were huge. That motor is rated at 390 hp and it only dynoed at 275 rwhp with 4.88 gears.

On our Miata powered Bronco we had to run low gears since the motor only makes 170 hp and 140 lbtq. So the 5.13's multiply the load being sent to the 37" tires.

With 442 lbtq. being made from the EcoDiesel there is no need to lower gears. 99.999% of people will be just fine with factory gear sets even running 40"s.

 

[TheSolarWizard]

There’s zero chance that on 40” tires that it would drive in any any way like it did stock without regearing

 

[ssteve]

Heard nitro gears will be offering 4.30’s. With 38’s-40’s I think that would be about perfect.

 

[JC JT]

Heard nitro gears will be offering 4.30’s. With 38’s-40’s I think that would be about perfect.

that would be good for 37s 456 for 38+ to 40+

 

[Dyzard]

Heard nitro gears will be offering 4.30’s. With 38’s-40’s I think that would be about perfect.

is there any word on these axles? been planning my build and from what im seeing these 4:30's would be pretty much perfect all rounder for 37's but dont see them on nitro's site

 

[txred38]

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

How has this held up for you over time? Did you swap axles or stay with this? I have the diesel with 40x15.50r20 tires and I’m try to decide what to regear to. Are the RPMs too much with 4.88 for the engine?

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