Rubi, 5.13s, auto tranny w/ 37s... anybody running this?

JLRubiMD

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I've read about plenty of people running 4.88s in the 8 sp auto and 37s, everyone seems happy, but I'm wondering about 5.13s. It'll save me a regear later on down the road but I'm pretty happy with my 37s and will be keeping them until the tread is gone, which will be at least a few years. Not overly concerned about fuel economy.

Because my tires actually measure 36" with the Jeep on the ground, 5.13s would bump me into the blue "power/towing, reduced MPG" column. Sounds fun. Really just looking for some feedback if anyone has this set up.

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Bearded_Dragon

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Lots of people have. Jump to 3:10 in the video:

 

spies

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Lots of people have. Jump to 3:10 in the video:
Ah yes a Chairman Oh video, this will surely be accurate and informative :facepalm:

Let's do some quick math, 4th gear is 1.67:1, 5th gear is 1.28:1. Difference of -0.39. 4.10 to 5.13 is a difference of +1.03. So you actually have MORE of a mechanical advantage in 5th gear with 5.13s than you do in 4th gear with 4.10s.
 

gsxr310

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Are you planning on running bigger than 37's later down the road?
 

LunchBoxFab

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while not an auto, i still chose 5.13s as well for my 36.5" KM3s - i should have them installed in a couple weeks.

i hope to report back with great news
 

UKCATS

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What speed is the graph in the OP predicated on? If it is 70mph, like most of these graphs are, it isn’t accurate. I’m running 35’s, 4.88 gears, and a 8 speed auto, and I’m only turning around 2350-2400rpm at 70mph.
 

Andrew05LJR

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Most of the graphs are based on 1:1 final drive as well. So there will be variances between the graph and actual real world experience

Edit: Here are your gear ratios. Maybe someone good at math can help you calculate what you'd be looking at compared to the graph

Gear Ratios:
  • 1st - 4.71
  • 2nd - 3.13
  • 3rd - 2.10
  • 4th - 1.67
  • 5th - 1.28
  • 6th - 1.00
  • 7th - 0.84
  • 8th - 0.67
  • Reverse - 3.53
 

Bearded_Dragon

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Ah yes a Chairman Oh video, this will surely be accurate and informative :facepalm:

Let's do some quick math, 4th gear is 1.67:1, 5th gear is 1.28:1. Difference of -0.39. 4.10 to 5.13 is a difference of +1.03. So you actually have MORE of a mechanical advantage in 5th gear with 5.13s than you do in 4th gear with 4.10s.
But doesn't it also matter where the RPMs are? Say 4.10s in 4th gear kicks down to 5000 RPMs, while 5th gear with 5.13s kicks down to 4000?
 

UKCATS

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Ah yes a Chairman Oh video, this will surely be accurate and informative :facepalm:
Chairman Oh. Lol! A herpes sore on the Jeep community since at least 2007.
 

spies

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Chairman Oh. Lol! A herpes sore on the Jeep community since at least 2007.
Well thankfully his forums are ghost towns so his reign of terror is over. Jeeps with radius arms have started coming out of the shadows, picking up the shattered pieces, and rebuilding their lives.

Now with that said I need to eat some crow here point out that my previous post is incorrect and Edwin was right. I added the gear ratios together, they are supposed to be multiplied. 4.10*1.67 is a 6.85:1 advantage, 5.13*1.28 is a 6.56:1 advantage. So your jeep will feel roughly 5% more powerful in 4th gear with 4.10s than it will in 5th gear with 5.13s, all else being equal.

And to actually help OP with his question, I would always pick the lowest gearing (numerically higher) possible if you aren't concerned about fuel economy or strength. As far as strength goes, I wouldn't run an 8 tooth rear pinion gear, but that rules out anything 4.88 and above. And there is a lot of debate about the relationship between pinion gear size/tooth count and strength anyway, I just have my beliefs because of what I've seen fail and not fail.
 
OP
JLRubiMD

JLRubiMD

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Thanks for the replies. I remember seeing that video a while back, forgot he had 5.13s. I try to stay clear of his videos...

I ran the math using my 36" actual tire height. I ran both the predicted RPM and predicted MPH to make sure they agreed with each other. On paper at least, it's not as big of a difference as I was expecting between 4.88s and 5.13s.

At 70 mph in 8th gear: 4.10s come in at 1795 rpm (goodbye 8th gear...)
4.88s come in at 2136 rpm (agrees with UKCATS RPMs adjusted for tire size)
5.13s come in at 2245 rpm

Just bear in mind these are for my "37 inch" tires that actually measure 36". My old "35 inch" tires measured 33.5", and actual size is what counts, at least that's what she said.

But I agree that calculations like these don't line up with the real world, so I'm just looking to get some input from people running this setup, mainly in regards to if they would do it again. Eventually I'll go bigger on the tires, but it'll likely be a few years.

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twisty

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Well thankfully his forums are ghost towns so his reign of terror is over. Jeeps with radius arms have started coming out of the shadows, picking up the shattered pieces, and rebuilding their lives.

Now with that said I need to eat some crow here point out that my previous post is incorrect and Edwin was right. I added the gear ratios together, they are supposed to be multiplied. 4.10*1.67 is a 6.85:1 advantage, 5.13*1.28 is a 6.56:1 advantage. So your jeep will feel roughly 5% more powerful in 4th gear with 4.10s than it will in 5th gear with 5.13s, all else being equal.

And to actually help OP with his question, I would always pick the lowest gearing (numerically higher) possible if you aren't concerned about fuel economy or strength. As far as strength goes, I wouldn't run an 8 tooth rear pinion gear, but that rules out anything 4.88 and above. And there is a lot of debate about the relationship between pinion gear size/tooth count and strength anyway, I just have my beliefs because of what I've seen fail and not fail.
Good stuff. Curious to hear what you have seen fail was with a big motor along with big tires?

I have been debating going with a heavier duty axle and lower gears but dana 60s seem overkill to me. Built 44s was what I was leaning towards since they are likely way more BUILT then factory. I was wondering about pinion size though, not so much for the front but the rear. The bummer with 44s is they dont make them (at least dynatrac for the JL) with locking hubs and that's what I want.
 

UKCATS

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Good stuff. Curious to hear what you have seen fail was with a big motor along with big tires?

I have been debating going with a heavier duty axle and lower gears but dana 60s seem overkill to me. Built 44s was what I was leaning towards since they are likely way more BUILT then factory. I was wondering about pinion size though, not so much for the front but the rear. The bummer with 44s is they dont make them (at least dynatrac for the JL) with locking hubs and that's what I want.
Dynatrac makes a D44/60 hybrid. D44 center section with D60 outters. Not sure if compatible with the JL, but you could call them and ask.
 

twisty

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Dynatrac makes a D44/60 hybrid. D44 center section with D60 outters. Not sure if compatible with the JL, but you could call them and ask.
Yeah I did. They said that will be the last thing they work on production-wise. Plus it's almost as heavy as a 60 and costs about the same too. At least per the dynatrac guy I spoke too.
 

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