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Thoughts on running 5.38 gears in a jku sport with 37 tires.
here is th current set up,
beefed up Dana 30
stock Dana 44
pros:
cons:
help with feedback.
thanks
Here are some charts, don't where they originally originated from, but, make sure you match your engine & tranny to the proper chart..
 

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Assuming you can find a 5.38 gear set for a D30 (I don't think you'll be able to), the gear set will be really weak. Just something to consider. Couple that with higher engine RPMs to increase torque transfer to the wheels, and you've got a ticking time bomb. IMO, its not a question of if you'll shred a ring gear, but when.
 

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Anyone know if the charts for the 2012's are labelled correctly? Auto's now have higher rpm's than 6spds?

And do the 2012 auto's have an OD Off button like the pre-12's?

If so, you might consider gearing lower (numerically) and just pressing the button.
Ex. (using the 2012 auto chart, and 36" actual height for 37" tires)
5.38 @ 70 = 2918 rpm
4.56 @ 70 = 2474 rpm (4.56 OD On)
2474 / .83 = 2980 rpm (4.56 OD Off)

Anyway, if there is an OD Off function, and if strength is a concern, it's something to think about.
 

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I am running 5.38 with 37's and am loving it. I do have a 4:1 Transfer case and D44 front and rear. But that still is same gearing in 2/4H. I still wish that 1st gear in the tranny was a bit lower for 2H stuff tho. I haven't had the opportunity to use 4L in technical stuff with these gears yet; but they should work great. Plenty slow in lower gears, and fast enough in the higher gears.
 

· The Wheelin Man's Friend
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I have a 2012 with 3.73s, auto, 35s and it runs just fine down the highway and around town.

I would not go deeper than 4.88 for 37s on the Pentastar.

The Pentatar = we aren't in Kansas anymore Toto! It is not the dog that the 3.8 and this should always be verified before we go giving gear advice.

Marcus
 

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This conversation (and my previous experience with my XJ with 4.88s and 35" tires) make me wonder if I'll like my 35" tires and 4.88 gears.

(shocked that the 3.21 gears and 35" tires are as "OK" as they are.... definitely need a gear change but not sure if 4.56 or 4.10 would be better....I'm thinking the 4.88/35" will be similar to driving a stock Rubicon with 4.10/32")
 

· The Wheelin Man's Friend
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This conversation (and my previous experience with my XJ with 4.88s and 35" tires) make me wonder if I'll like my 35" tires and 4.88 gears.

(shocked that the 3.21 gears and 35" tires are as "OK" as they are.... definitely need a gear change but not sure if 4.56 or 4.10 would be better....I'm thinking the 4.88/35" will be similar to driving a stock Rubicon with 4.10/32")
Like I mentioned, I'm running a JKU with heavy 35" Toyos and 3.73s right now and it's not bad at all. I can hammer it on the highway and it'll get out of it's own way for sure. We have a toll road with an 85 MPH speed limit and I can cruise down that thing without any problems.

I think 4.10s or 4.56 with 35s would be ideal. I'd only go to 4.56 because of the cost prohibitive 4.10s. (You have to use factory issue gears and they are double the cost of aftermarket options.)

Marcus
 

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You can go with 4.88s with a '12+ JK and be good. I jusy went from 4.88s to 5.38s on my 09 Rubi and it's great. just did an 800 mile round trip to Denver from SW Colorado and it cruised 65 at 2200, got about 17.5 mpg overall.
 

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You can go with 4.88s with a '12+ JK and be good. I jusy went from 4.88s to 5.38s on my 09 Rubi and it's great. just did an 800 mile round trip to Denver from SW Colorado and it cruised 65 at 2200, got about 17.5 mpg overall.
Cheating with Spyntec hubs, I'm jealous for sure
 

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I'm thinking 5.38 gears with 37" tires in a 3.6l aren't as necessary as they would be in the 3.8l? I'm thinking 4.88 might be fine with 37"? 5.13 max, IMHO. I've driven overgeared rigs before. They suck down the fuel, tend to run hot and just are generally as miserable as something that's way undergeared.


I also think it may have to do with actual tire height as well. Some 37" are 36" and some are every bit of 37"
 
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