Gears for 37" tires

[Oscar Indy]

So you said with 4.10 gears, yours won’t hold 8th with a head wind. My opinion is that if you’re going to bite the bullet and regear, you go as deep as you can tolerate. The deeper gear will let the engine run lean mixture and full ignition advance more, which translates into offsetting any economy lost vs a smaller gear ratio.

Yep. And with 4.88 it holds 8th fine. If I was gonna do 5.13.... I'd have just skipped regearing. Hard to get 2k back at the pump.

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[Reasons]

If going to 37s without plans to re-gear from 4.10s, would choosing the lightest wheel + tire combo (Venomrex? + BFG K02) possibly work?

I have plans to go 35 since I daily my Gladiator. My daily commute is a mix of highway and street, averaging 18mpg now. I do see 8th gear but only on the highway. However, I parked next to a JTR with 37s today and I really do like the look a lot more than 35s. My head says to go 35 but my heart wants to go 37s now. I have zero intentions of re-gearing and I dont want to lose 8th gear.

 

[Oscar Indy]

If going to 37s without plans to re-gear from 4.10s, would choosing the lightest wheel + tire combo (Venomrex? + BFG K02) possibly work?

I have plans to go 35 since I daily my Gladiator. My daily commute is a mix of highway and street, averaging 18mpg now. I do see 8th gear but only on the highway. However, I parked next to a JTR with 37s today and I really do like the look a lot more than 35s. My head says to go 35 but my heart wants to go 37s now. I have zero intentions of re-gearing and I dont want to lose 8th gear.

A 37 in k02 is actually a 36 so you might be much better off

 

[SleepyJeep]

Does anyone have an article or a technical paper that explains the ratio, gearing and re-gearing and the rear differentials explained? I don't entirely understand it and I don't mind technical papers either cuz I have an engineering background and would understand it... I am a n00b when it comes to this...

I have been contemplating 35s Vs 37s BFg KM3s... Mostly I might go to 35s but some of you have recommended me to go to 37 cuz it is effectively 36... LOL this is going to be my hardest decision cuz I don't plan on rock crawling and mostly overlanding... re-gearing is gonna cost additional $$

 

[Reasons]

I am in the same boat as you. After thinking long and hard about it, when I go camping with the family I will be pushing my Rubicon very near its max payload capacity. Which, is why I'm staying with 35" tires. Re-gearing is something I dont want to do.

But damn, a Rubicon with 37" looks so good with its higher fender flares.

 

[WhatExit?]

We know the Gladiator can run 37's without a gear change so the question is do you want to do it? The Gladiator will handle the 37's but you'll see more time in lower gears as a result.

 

[Factoid]

Oscar Indy has done a very nice job of synthesizing the data and cutting to the chase in this thread. To deconstruct his points, you will deal with conversion factors around ratios, diameters and circumferences. Best to use the simplified data, but a google search will give you the overwhelming data you seek!

 

[ShadowsPapa]

Does anyone have an article or a technical paper that explains the ratio, gearing and re-gearing and the rear differentials explained? I don't entirely understand it and I don't mind technical papers either cuz I have an engineering background and would understand it... I am a n00b when it comes to this...

I have been contemplating 35s Vs 37s BFg KM3s... Mostly I might go to 35s but some of you have recommended me to go to 37 cuz it is effectively 36... LOL this is going to be my hardest decision cuz I don't plan on rock crawling and mostly overlanding... re-gearing is gonna cost additional $$

Just some basics - gear ratios can be tricky because if someone says "lower ratio" it's actually a bigger number numerically. So the number going higher is a lower ratio, number going smaller is a higher ratio.
3.73:1 is a higher ratio than 4.10:1
It's the teeth of the pinion compared to those on the ring gear. If there are 40 teeth on the ring gear and 10 in the pinion, it's a 4 to 1 as the pinion will turn 4 times for every turn of the ring gear. ( expressed 4:1 )

So on an Overland the drive shaft will turn 3.73 times for every 1 turn of the axles (assume they are locked together for simplicity)
A Rubicon drive shaft will turn 4.1 times for every 1 turn of the axle (pinion turns 4.1 times for every 1 of the ring gear)

When you re-gear you change the pinion and the ring gear. They are matched sets. A lower gear ratio means the pinion is smaller diameter.
To get the ring gear to engage with the smaller pinion since the teeth would be further away, they make the ring gear thicker to move the ring gear closer to the pinion to keep mesh.
They will only make a ring gear so thin or so thick.
Get past a certain point and you have to swap the carrier - the part the ring gear bolts to.
For the Dana 44 you have to change the carrier to go from a 3.73 ratio to a 4.10 because the ring gear has to be moved closer to the pinion to keep things meshed.

Then you enter into the ratios of each transmission gear. I believe direct or 1:1 is 6th gear in these, 7th and 8th both overdrive gears.

Larger tires, fewer revolutions per mile, slows the engine RPM - fewer engine revolutions for each mile traveled. Good for MPG but you then get out of the sweet spot for power and lug the engine - or the transmission won't upshift.

In these pics are the carrier and the ring gear still bolted to it - I'm rebuilding with new bearings.

I have pics somewhere of the limited slip differentials I have rebuilt but won't bother unless someone wants to see the disk type limited slip.

 

[SleepyJeep]

We know the Gladiator can run 37's without a gear change so the question is do you want to do it? The Gladiator will handle the 37's but you'll see more time in lower gears as a result.

Pardon my ignorance but lower gears in highways means less mileage right? idk how ppl are getting 19+ mpg, I'm averaging at 16.5mpg LOL

 

[WhatExit?]

Pardon my ignorance but lower gears in highways means less mileage right? idk how ppl are getting 19+ mpg, I'm averaging at 16.5mpg LOL

The gear changes ("re-gearing") we're talking about are used to compensate for larger tires in order to get the JEEP back to factory-like performance (transmission/RPMs) but certainly not MPGs

 

[ShadowsPapa]

Pardon my ignorance but lower gears in highways means less mileage right? idk how ppl are getting 19+ mpg, I'm averaging at 16.5mpg LOL

Yes and no. Frankly, running an engine too slow on the highway drops it below the peak power or performance band and can actually hurt MPG.
It's sort of like what I ran into in the 70s and 80s when we, as mechanics, compared MPG of some V8s to I6s - and found the V8 got better mileage - because it was not lugged or worked hard, the load was a good match for the engine while the 6 cyl was being over-worked and did poorly in MPG.
Run the gear ratio up too high and you lug the engine, you work it harder, you have to give it more throttle.
There are sweet spots for every engine, depending on design, cam and other factors. I should post the dyno sheets from my built 360.

Anyway, if the engine is running too slow you don't keep it breathing at peak efficiency. Air flow rates drop, you don't pack the cylinders as well, combustion pressures are down, a lot of factors.
Every cam grind has a peak power band area, heads flow differently, it gets complex.
But bottom line - if you run an engine too slow, you hurt power and mpg. Run it too fast and you waste that power and drop mpg.
The gear changes talked about here are to keep the engine RPM up when the tire size increases as when you go from say a stock 33" tire to a 37" or greater, you slow the engine enough you drop below the power band and hurt power because it's running too slow to produce power.
"deeper gears" or a higher numerical ratio - which is a lower gear ratio, brings the engine speed back up. So you can run taller tires to get that Jeep off-road stuff back and speed the engine back up with the gear change for the torque you need to crawl that steep grade.
Machinery's handbook has a lot on gear ratios and such. (it's a paid-for copyrighted fat book so like can't find the whole thing online for free)

 

[Daniel.cox8]

I got 4.88 in my gladiator finished break in period and at 70 75 I hold 8th at 2100 rpm no more down shifting or anything I'm happy with 4.88

 

[WhatExit?]

I got 4.88 in my gladiator finished break in period and at 70 75 I hold 8th at 2100 rpm no more down shifting or anything I'm happy with 4.88

The End

 

[UKCATS]

I got 4.88 in my gladiator finished break in period and at 70 75 I hold 8th at 2100 rpm no more down shifting or anything I'm happy with 4.88

That’s the ratio I’m re-gearing the wife’s Gladiator to. She is running 37” Trail Grapplers, 8 speed auto. I am running 4.88’s in my 2 door JL with 37” Ridge Grapplers, 8 speed auto, and it does well. If I had a manual with 37’s, I would do 5.13’s.

 

[TennesseePA]

I linked a video earlier in the thread about trying to do 80 on 37s. The 4.10s don't have enough torque to hold 8th and 80mph. it'll do it perfectly flat and downhill. any headwind or SLIGHT incline and it'll gear down to the 7th or 6th depending on severity. I tried locking it into 8th and seeing what it would do. 71 is all it'll hold at my altitude.

I'd almost bet my house that the 3.6 cannot hold 80 on a hill even with 5.13 s in it. It is a well known fact that the 3.6 is not a torque biased engine it is a horsepower biased engine. It takes a defined amount of work to push a vehicle up a hill. Your RPM will be in the same range as mine whether you run 2.73 or 5.13. The only difference is where in the transmission that defined amount of work takes place. The plain and simple argument is that physics rule the day and if you want to pull 8th on a long hill you'll have to go a whole lot deeper than 5.13.

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