3.6L, 8 AT, 37”: 4.88 or 5.13?

[JeepinJason33]

I've never weighed my Jeep but, she's not light by any means lol. And yes, I'm right at sea level here on the coast, great info. Thank you.

I have added bumpers front and rear, winch, side bars, full Metal Cloak lift and under armor, and Beadlocks with BFG 37"s. I took it to a truck stop and was surprised to see how much weigh this added. At the time I had the soft top on instead of the hard top.

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[Arrowhead]

As I write this I realize that there are a lot of threads about this topic, but this thread has stayed on topic and people with real world experiences are sharing their opinions. Everyone has unique needs and expectations, so I will share mine and hopefully it will help to get valuable opinions. I have a 2 door so it is lighter weight than most. I do have a heavy wheel tire combo ( 134#). My jeep is not a daily driver but I live in rural are that requires highway speeds to get anywhere. My thoughts are that I have run the stock 4:11 ratio for 5000 miles and it is not terrible, but I am looking forward to a gear change. Maybe 4:88 would be enough and have stronger pinion than 5:13? Or will I wish I had 5:13 except when I need to travel 75+ mph? I am not a speed demon, but I also need to stay out of the way of interstate traffic that tends to run 80ish mph around here. The short wheelbase of a 2 door on 37” tires can be a bit “twitchy” above 75 mph.

I have 37's on my 2dr and swapped the 4:10's (Sport with Rubi axle swap) to 5:13 before my cross country trip to Moab and Colorado this summer (4k miles round trip from NY). I was very happy with the setup. Some of the interstates were 80 mph out west though. I never felt unstable at that speed - BUT even holding 8th gear the fuel milage tanks at that speed. So I backed off by choice, not so much to save $ but to extend the range and not have to fill up so often.

 

[jdhunt0]

I just drove the 90 mile trip home on my newly installed 4:88 gears. I am still on 35s for now, but I am happy with how they are on the road. RPM at 75 in 8th is not much higher than what I was seeing with the stock 3.45s in 6th.

The rear diff was very hot when I got home but oil level was fine. I did put it in 4L once I got on a gravel road and the service 4WD light came on. Not sure what that’s about.

I’ll test it off road as soon as I get my 500 miles in.

 

[lslblues]

Add me to the happy with 5.13s club. 2021 JLUR 3.6 with 37s, aftermarket front/rear bumpers, and metal cloak skid plates. Big improvement in acceleration, mpg, and shifting. It just shifted the way it should and know lives in 8th gear on the highway. Went back and forth between 4.88 and 5.13 and who knows, I may have been happy with 4.88s as well. But I am definitely happy with the 5.13s.

 

[Wraith]

I was planning to do 4.88s but after speaking with several people on 37s it seems 5.13s are the way to go. Glad I found this thread. Spoke to the installer this morning and switched my order from Spicer 4.88s to Spicer 5.13s. Looking forward to getting it done!

 

[CarbonSteel]

I did the full gamut:

33's + 4.10
35's + 4.10
35's + 4.88
37's + 4.88
37's + 5.13

35's + 4.88 & 37's + 5.13 are the best as far as performance. Can you get away with less? Sure, but with a 3.6L, altitude, load, wind, and hills/mountains are not your friends when you are under geared and it is in no way optimum.

Love my 37/5.13 combination--into the flat part of the torque curve at highway speeds, no dragging out of shifts, no loss of 7th and 8th, 17-18MPG at speeds less than 75; what's not to love?

 

[CarbonSteel]

I just drove the 90 mile trip home on my newly installed 4:88 gears. I am still on 35s for now, but I am happy with how they are on the road. RPM at 75 in 8th is not much higher than what I was seeing with the stock 3.45s in 6th.

The rear diff was very hot when I got home but oil level was fine. I did put it in 4L once I got on a gravel road and the service 4WD light came on. Not sure what that’s about.

I’ll test it off road as soon as I get my 500 miles in.

Check the locker electrical connectors on the axle -- they may not be fully engaged.

 

[Wraith]

I did the full gamut:

33's + 4.10
35's + 4.10
35's + 4.88
37's + 4.88
37's + 5.13

35's + 4.88 & 37's + 5.13 are the best as far as performance. Can you get away with less? Sure, but with a 3.6L, altitude, load, wind, and hills/mountains are not your friends when you are under geared and it is in no way optimum.

Love my 37/5.13 combination--into the flat part of the torque curve at highway speeds, no dragging out of shifts, no loss of 7th and 8th, 17-18MPG at speeds less than 75; what's not to love?

I was pretty set on 4. 88s but after reading through this thread and knowing how I use my Jeep (travel to the mountains, etc.) the 5.13s made the most sense. Thankfully the installer said no problem on switching gear sets so in a few weeks I should be regeared. Really looking forward to getting it done.

 

[Chocolate Thunder]

I don’t think that either 4.88 or 5.13 are “wrong” for 37s, depending on what you want to achieve and your circumstances. For me 4.88 has been great. I don’t think I’d be disappointed if I’d gone 5.13 either. I live at sea level and value highway cruising in my 8AT. But they were also great on a 2500 mile road trip and climbing mountain passes in Colorado.

 

[Redbaron73]

I don’t think that either 4.88 or 5.13 are “wrong” for 37s, depending on what you want to achieve and your circumstances. For me 4.88 has been great. I don’t think I’d be disappointed if I’d gone 5.13 either. I live at sea level and value highway cruising in my 8AT. But they were also great on a 2500 mile road trip and climbing mountain passes in Colorado.

Same for me. I spend most of my highway driving in the dfw metroplex, and most mountain driving at low speeds.

I have been happy. 8th is a bit less than would be with 5.13, but not going to redo it.

 

[J0E]

You don't have 37s do you? The gear calculators are theoretical.

LOL, it's 7th grade arithmetic. See my https://bt39.com/GearWheelSpeed

This doesn’t account for the rolling mass of the new tires though. Unless you have unlimited torque it makes a big difference. I would go 5:13 with 37s for that reason.

Rolling mass? I think you might be talking about rotational inertia, which is insignificant compared to the mass of the vehicle. Rotational inertia is a scalar value of how difficult it is to change the rotational velocity, and the change in velocity in high gears is very low. Mag wheels were invented because dragsters had large wheels (big Radius) and enormous change in velocity, therefore the lower mass significantly lowered the rotational inertia.

I'm on 37's with stock wheels, stock gears, JLR 2.0T. Mine is a dedicated crawler, but I do drive to the trails so I won't waste the money for lower (and weaker) gears.

I don't understand the obsession with hitting 8th gear. I hit 8th gear every weekend, albeit in low range with the 4:1 ruby xfer case.

 

[CarbonSteel]

I'm on 37's with stock wheels, stock gears, JLR 2.0T. Mine is a dedicated crawler, but I do drive to the trails so I won't waste the money for lower (and weaker) gears.

I don't understand the obsession with hitting 8th gear. I hit 8th gear every weekend, albeit in low range with the 4:1 ruby xfer case.

The 2.0T is less impacted by under gearing (as is the 3.0TD) because those engines make enough torque and at much lower RPMs versus a 3.6L

While I cannot speak for others, it is not an obsession about hitting 8th gear, it is about a properly running vehicle that is not lugging on the highway, dragging out gear shifts, constantly hunting for the "right" gear, and experiencing a substantial loss in MPG which will absolutely happen with a 3.6L/4.10s/37s.

As for weaker gears, just how many examples of broken R&P have we seen in the JL series with Dana 44 Advanteks and of those how many were expected due to the setup/use case? With the internet amplification that happens these days, it would be posted ad nauseum if it were regularly happening.

I am of the opinion that a 5.38 gear is strong enough and everything higher than that is that much stronger. It is those who take their Jeeps far beyond reasonable limits for the design that break stuff--e.g. running 40s on an axle with a R&P that is smaller than one found under a 1980s Ford Mustang. Which given that, is a testament to the design and metallurgy of the Advanteks as there are those running 40s on them.

 

[Crowbar]

Working with a dealer to put an order together for a JLUR and I was surprised that I could add 4.88's for 895.00. Had initially been planning to re-gear to 5.13's for 37's but if 4.88's are available oem for that price I may go 4.88's. I have 5.13's in my heavy JTR with 37's and beadlocks and love the 5.13's. Maybe the JLUR will be enough lighter than the JTR that 4.88's will be satisfactory? I've seen too many posts where people have actually re-geared from 4.88's to 5.13's and certainly don't want to go that route.

 

[DanW]

Working with a dealer to put an order together for a JLUR and I was surprised that I could add 4.88's for 895.00. Had initially been planning to re-gear to 5.13's for 37's but if 4.88's are available oem for that price I may go 4.88's. I have 5.13's in my heavy JTR with 37's and beadlocks and love the 5.13's. Maybe the JLUR will be enough lighter than the JTR that 4.88's will be satisfactory? I've seen too many posts where people have actually re-geared from 4.88's to 5.13's and certainly don't want to go that route.

I'm running 4.56 with 35's and I think it is perfect. I'd predict 4.88 with 37's would be just fine.

 

[Crowbar]

I'm running 4.56 with 35's and I think it is perfect. I'd predict 4.88 with 37's would be just fine.

What altitude and terrain are you in. I'm at 4000 plus feet, mountains and heat.

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