Gears for 37" tires

[Flux]

or I could just come down to Austin and we could put your rig to the test in the hill country.

I’m going to need to find what OHV parks are around here.

Sponsored

 

[19JL]

Another key issue I don't think I saw mentioned is pinion size. Not sure if anyone has a photo of the pinion size of a 4.10 pinion vs a 4.88 and 5.13, but from what I recall on the JK, the 5.13 pinion is considerably smaller than the 4.10 while the 4.88 is slightly smaller....and with turning larger tires, especially through mud, deep snow, and obstacles, it can be a weakpoint you didn't want. So, is it worth the little bit of extra pep? I guess depends on who you ask.

Here’s a 5.13 pinion vs 4.10, there’s a noticeable difference in size.

 

[Duke56]

Are you saying that the failure rate goes up due to smaller pinion diameter?
Is there any data showing the failure rate?

 

[19JL]

Are you saying that the failure rate goes up due to smaller pinion diameter?
Is there any data showing the failure rate?

I’m not saying that at all I just posted a photo that someone asked about. I was just saying there is a noticeable size difference and less teeth on the 5.13s. In my out on the trail life experience I see a lot more 5.13, 5.38 ring and pinions break over 4.10s. But maybe that’s because most people that wheel have already regeared for better crawl ratio.

 

[MrKnowitall]

I’m not saying that at all I just posted a photo that someone asked about. I was just saying there is a noticeable size difference and less teeth on the 5.13s. In my out on the trail life experience I see a lot more 5.13, 5.38 ring and pinions break over 4.10s. But maybe that’s because most people that wheel have already regeared for better crawl ratio.

There is probably strong correlation to gears deeper than ~5 and tires that are simply to big for the axles and/or power upgrades.
Personally, I dislike the Rubicon 4:1 lowrange in midwestern trails, preferring the 2.72. Chances are, if you’re not breaking 4.10 gears with a Rocktrac case, 4.88s will be fine for me, and substantially better than 3.73 in the road.

 

[MrKnowitall]

Then again, maybe the 4:1 low range is the reason the JL and JT get such a winky clutch- keep the axles in one piece.

 

[wvyankee2]

I have 35's on my Overland and am installing 488's next week. The 3.73 has noticeable power loss on Interstate. With the Auto8 I will have better crawl ratio and get better use of my 7th and 8th gear on the Highway. I'm sure it will improve fuel mileage also.

 

[Alabama Mud Machine]

I have 35's on my Overland and am installing 488's next week. The 3.73 has noticeable power loss on Interstate. With the Auto8 I will have better crawl ratio and get better use of my 7th and 8th gear on the Highway. I'm sure it will improve fuel mileage also.

4.10’s or 4.30’s would be about perfect for 35’s, and 4.56’s would probably be about the deepest I’d go for 35’s, but if you plan on going 37’s in the near future, then the 4.88’s should suit you well.

 

[BreakFixRepeat]

This has probably been talked to death on this thread. But i have a rubicon all stock for now. Plan is to lift and run 37s. Always told myself gears was gonna happen when i did this. Well then i drove a sport with 3.73s on 37s. I was really impressed with how pepping it still was compared to mine. Now im pretty sure mine would have still run away from it without breaking a sweat, but mine is 4.10s on stock 33s. Overall i have decided that im gonna do the lift and tires and not worry about gears. If one with 3.73s did that well then im sure mine with 4.10s is gonna be just fine. Just my opinion.

 

[Mr. Wuf]

If I had 4.10s, I’d probably skip the regear. My 3.73s feel a bit sluggish with 37s. Drivable, but far from great. I’ll be going to 4.88s later this summer.

 

[Alabama Mud Machine]

If the JT had a bigger engine, or more power (somewhere in the 350 HP range), the 4.10's would be ok. Because gearing-wise it feels great. It's NOT sluggish at all, surprisingly peppy from a dead-stop, honestly, for having that weak of an engine...Especially with the iDrive and the Tazer adjustments. But if it had more power, I think the 4.10's would be adequate for 37's in the JTR because of the 4:1 transfer case when off-roading. But on the street in the mid-to-upper RPM ranges is where I feel the most loss.

My Cummins has 4.10's and 38's, and makes over 550-600 RWHP and over 1,100 RWTQ. I lifted that truck at 2,220 miles on the odometer (basically a month or 2 after purchase)... And I have never felt under-geared in that truck running 38x15.50's on heavy 20x10 wheels. I knew I was going to be lifting it and going with bigger tires, so I ordered it with the factory 4.10 gears and lockers option (back when you could still do that, before rich city boys turned truck ownership into a status symbol, and no longer for the blue collar folks).

 

[Flux]

Well, after a few weeks with the 5.13s in, I'm very pleased with the outcome. There was never any real concern for low speed driving due to the 8sp gearing, but it makes a huge difference above 50 mph or so. Additionally, my mpg has gone up about 2mpg so far and the more miles I put on it, it keeps climbing due to averaging on the speedo. I'll ignore the input from others who have gone up in tire size without regearing and trying to claim their Jeeps don't need it since they, apparently, have magical unicorn jeeps that defy the basic laws of physics. For the rest of us commoners, regearing will put your engine/trans back in the correct RPM range to account for the larger tires and will in essence, "re-center the fulcrum". Keep in mind that attempting a direct RPM conversion for gearing won't be adequate since you have added considerable unsprung weight, rolling resistance, and increased wind resistance. I got the Yukon gears from Northridge for less than $900 including master install kits, and install near me was 1K for both axles.

 

[MrKnowitall]

Well, after a few weeks with the 5.13s in, I'm very pleased with the outcome. There was never any real concern for low speed driving due to the 8sp gearing, but it makes a huge difference above 50 mph or so. Additionally, my mpg has gone up about 2mpg so far and the more miles I put on it, it keeps climbing due to averaging on the speedo. Keep in mind that attempting to a direct RPM conversion for gearing won't be adequate since you have added considerable unsprung weight, rolling resistance, and increased wind resistance.

Not surprising that fuel econ actually improved- if you lug the engine, it’ll constantly be pulling ignition timing and running rich mixture.
Because of all the added drag of running big tires, I still like multiplying the original gear by the square of tire circumference increase.
3.73*(660/562)^2=5.14
Or just look at what the highest tolerable cruising rpm would be. If you want to hold 8th gear at 75-80, you need to turn at least 2500.
37s turn ~562rev/mi, 8th=2/3
562*5.13*(80/60)*(2/3)=2563rpm at 80.

 

[Flux]

Not surprising that fuel econ actually improved- if you lug the engine, it’ll constantly be pulling ignition timing and running rich mixture.
Because of all the added drag of running big tires, I still like multiplying the original gear by the square of tire circumference increase.
3.73*(660/562)^2=5.14
Or just look at what the highest tolerable cruising rpm would be. If you want to hold 8th gear at 75-80, you need to turn at least 2500.
37s turn ~562rev/mi, 8th=2/3
562*5.13*(80/60)*(2/3)=2563rpm at 80.

You are right on. She seems quite content with the new gears and RPM range. This was about 100 miles after install. The MPG average had been around 12.6 with the 4.11 gears. I just hit 1100 miles and average is now around 14.8. If I actually drove more, it would probably be higher, but it'll get there soon enough. I live in Austin, so it's not mountainous, but not flat either. This picture was as I was going up an overpass.

 

[Alabama Mud Machine]

My JTR averages 14.5-15 with stock 4.10's and 37's after recalibration with the Tazer, and adding the iDrive unit. It was getting around 12.5 MPG 100% stock. I'd imagine with a 4.88 re-gearing, I'd be in the 16.5-17 MPG range.

Sponsored