37 vs 38’s and 4.88 vs 5.13

[Hemy5587]

It’s the same mentality people have when looking at full size trucks. “If I get a 6cyl instead of an 8cyl I’ll get better MPGs”. That rarely works because they have to work the 6cyl 25% harder to get the same result, so the mileage gains end up being minimal.

Now if you drive that 6cyl like a little old woman, yea you might get close to the higher mileage, but with real world activities it just doesn’t often work out that way.

Well... people have claimed I drive like a grandpa

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[tysongladiator]

I've got 37" Yoko X-AT with 5.38s. I previously had 4.88s. But when I added lockers, I went to 5.38s. I'm glad I did. I can get around 15mpg on the highway running 70mph. Not a huge difference between my friend's 5.13s. But the big difference is when I tow. I have about 5 trailers that I tow and when I tow my 20ft trailer with a tractor on it, I can still get around 14mpg on the highway doing 70mph.

I've learned over the past few decades that even though they may be from the same manufacturer, vehicles will react completely different to upgrades and modifications.

 

[Dano3190]

I'm extremely new at this (jeeps) matter fact my 25' Rubicon has 2100 miles. I don't have an ounce of care about gas mileage or driving fast I didn't buy this for that. But Texas highway speed limit is 75mph. SO after all the theorys and information what gathered is- 5.13 is shorter gears but with more gears. 4.88 is longer gears with less gears. I know its the same amount of gears but with 4.88 you dont see 8th as often. I dont seem to find any reason why for myself at least 5.13s and 38s Patagonias should even be questioned? NO crawling, no extreme offroad, just trails and beach days with 8-12hr drives to destinations. Anything passed that Im towing the rig there.

 

[gearhead22]

I'm extremely new at this (jeeps) matter fact my 25' Rubicon has 2100 miles. I don't have an ounce of care about gas mileage or driving fast I didn't buy this for that. But Texas highway speed limit is 75mph. SO after all the theorys and information what gathered is- 5.13 is shorter gears but with more gears. 4.88 is longer gears with less gears. I know its the same amount of gears but with 4.88 you dont see 8th as often. I dont seem to find any reason why for myself at least 5.13s and 38s Patagonias should even be questioned? NO crawling, no extreme offroad, just trails and beach days with 8-12hr drives to destinations. Anything passed that Im towing the rig there.

It depends on your location/driving terrain, driving habits, weight of the vehicle etc. 5.13’s will get you into 8th easier, but at 75 your rpm’s will be a bit higher. 4.88’s won’t get into 8th as easily, but you will be running lower rpm’s at 75. It’s really a personal preference. If you’re terrain is flat I doubt you’ll see much of a difference in holding 8th.

 

[Panthers65]

I'm extremely new at this (jeeps) matter fact my 25' Rubicon has 2100 miles. I don't have an ounce of care about gas mileage or driving fast I didn't buy this for that. But Texas highway speed limit is 75mph. SO after all the theorys and information what gathered is- 5.13 is shorter gears but with more gears. 4.88 is longer gears with less gears. I know its the same amount of gears but with 4.88 you dont see 8th as often. I dont seem to find any reason why for myself at least 5.13s and 38s Patagonias should even be questioned? NO crawling, no extreme offroad, just trails and beach days with 8-12hr drives to destinations. Anything passed that Im towing the rig there.

it’s a 4.87% difference between 4.88s and 5.13s, that’s the difference between 2200rpm and 2305rpm. Most people at 70mph can’t distinguish 100rpm difference, that’s why we have tachs.

with 38s imo I’d go 5.13s, but if you REALLY are never going to crawl, pull trailer, haul heavy loads, or basically treat your truck like a truck, the 4.88s would be fine.

then again if you aren’t planning on doing anything like that, factory 4.10s would probably be fine, you’ll just drive like grandpa Joe moving forward.

 

[Zachanadandy]

then again if you aren’t planning on doing anything like that, factory 4.10s would probably be fine, you’ll just drive like grandpa Joe moving forward.

Just out here doing grandpa Joe stuff. 4.10s and 37s. 1200 mile roadtrips every 4-6 weeks with the cruise control set at 85mph. Sees the 99mph governor in the dirt regularly. Towed a heavy trailer 600 miles with over 5k feet of elevation gain. Rock crawls ok (although the trail limo length and poor breakover aren't great for that so I'll usually take the JLUR if that's the plan). Regearing doesn't do anything a downshift can't, outside of 1st gear. 7th is the same final drive ratio with 4.10s as 8th with 5.13s. The rpm, engine load, torque at the wheels, etc are identical. But you see an 8 on the display so that's obviously 1 better than a 7...
I'm not saying 4.10s are optimal for 37s, but the truck still does truck stuff just fine and Jeep stuff better than a TJ or JK no matter what gearing you put in it. 4.88s are more than enough and you won't lose freeway fuel economy as much as you would with 5.13s.

 

[gearhead22]

What he said ^^

 

[Panthers65]

Don’t be so easily offended, it was just a joke. We all know you have a gooseneck in your truck and regularly haul 20klb trailers through the mountains in the middle of August.

 

[Zachanadandy]

Don’t be so easily offended, it was just a joke. We all know you have a gooseneck in your truck and regularly haul 20klb trailers through the mountains in the middle of August.

No matter how many times you post illogical nonsense that isn't supported by reality or the simple math of final drive ratios it still won't be true. The vast majority of JLs sold since 2018 have 32" tires and 3.45 gears. That's the equivalent gearing of 4.10s and 38s. You're Jeep will still do anything and everything it was designed to do with 4.10s and 37s period. Obviously it's taller, even less aerodynamic, and pushing bigger tires so it will never get the mpgs it would stock. It won't handle the same. None of that will change with a regear either. It could use a little more gearing to counteract the weight and aero changes, which is why in my experience 4.56s are perfect for 37s everywhere. Freeway, off road, towing, hills. The problem is that's such a small gear change it's hard to justify spending the money. I felt more of an improvement in acceleration, mpg, and overall driveability by adding a tune to the 2019 JLUR on 38s than I did with the regear. The tune was under $1k and the Jeep cut a full half second off is 0-60. The regear only cut 1/10th of a second...for $3k. Several others have measured 0-60 before and after regearing with the same results. Altitude and even weather will have more of an effect on 0-60 on any given day.

 

[Panthers65]

tldr

okay, 100 years of common automotive performance knowledge, but you’ve convinced me. I’m out

 

[Hemy5587]

I think we have established you can regear and be happy, not regear and be happy and be me with 3.73s and 37s and be happy. MPG, driving experience and general feel of the truck is all up to the interpretation of the driver and what you want out of your truck. OP let us know what you end up doing!

 

[Zachanadandy]

tldr

okay, 100 years of common automotive performance knowledge, but you’ve convinced me. I’m out

Look at the 1st gear ratio of every auto before the 8 speed. Simple math. The TJ needed duece and a half axles with their 6.72 ratio to match the same drive ratio of the 8 speed and 4.10s. Even the late model JK only had a 3.59 1st gear. They didn't come with the 5.38s they'd need to match the 4.10s with the 8 speed? The math hasn't changed in thousands of years and if you were properly applying that 100 years of automotive knowledge you'd understand that final drive ratio is all the engine sees for load and putting power to the ground period.

 

[Wheelin98TJ]

tldr

okay, 100 years of common automotive performance knowledge, but you’ve convinced me. I’m out

I used to reply to his gearing posts. It's not worth your effort.

Lots of nonsense to justify having stock gears with 37s.

He used to say his JL was slower 0-60 with deeper gears. Now it's 1/10th second faster.

...I felt more of an improvement in acceleration, mpg, and overall driveability by adding a tune to the 2019 JLUR on 38s than I did with the regear. The tune was under $1k and the Jeep cut a full half second off is 0-60. The regear only cut 1/10th of a second...for $3k...

...We geared our 2019 JLUR to 5.38s with 38s because the internet was convinced that was the thing to do. Lost 2mpg on average and 3 on the freeway. Lost .5s 0-60 as it had to shift 1 extra time to get there....

 

[Zachanadandy]

I used to reply to his gearing posts. It's not worth your effort.

Lots of nonsense to justify having stock gears with 37s.

He used to say his JL was slower 0-60 with deeper gears. Now it's 1/10th second faster.

2 different JLs, the 2.0t lost a half second as its redline is so low it had to shift an extra time to get to 60. Not really relevant to the JTs as there is no 2.0t option. The 3.6L didn't gain anything and neither did anyone else over there that actually measured. You'll get lots of "it rockets off the line after the deep gears" posts, but actually measured it doesn't. Shifting gears sooner, at lower speed and the same rpm might feel quicker... but it isn't. Facts matter. Final drive ratio is the only connection between the engine and the tires. It doesn't see any difference if you have 4.10s in 7th or 5.13s in 8th. Math doesn't lie. It wasn't the same JLUR, the v6 won't gain anything and the 2.0t will lose 0-60 with really deep gears and that's been measured and documented by numerous people. The 3.6L doesn't see any difference if it's a JK 5 speed auto with 5.38s or a JT/JL 8 speed with 4.10s. The final drive ratio in 1st gear is the same.

 

[Wheelin98TJ]

2 different JLs, the 2.0t lost a half second as its redline is so low it had to shift an extra time to get to 60. Not really relevant to the JTs as there is no 2.0t option. The 3.6L didn't gain anything and neither did anyone else over there that actually measured. You'll get lots of "it rockets off the line after the deep gears" posts, but actually measured it doesn't. Shifting gears sooner, at lower speed and the same rpm might feel quicker... but it isn't. Facts matter. Final drive ratio is the only connection between the engine and the tires. It doesn't see any difference if you have 4.10s in 7th or 5.13s in 8th. Math doesn't lie. It wasn't the same JLUR, the v6 won't gain anything and the 2.0t will lose 0-60 with really deep gears and that's been measured and documented by numerous people. The 3.6L doesn't see any difference if it's a JK 5 speed auto with 5.38s or a JT/JL 8 speed with 4.10s. The final drive ratio in 1st gear is the same.

You didn't mention that Jeep here:

A 5.7L hemi would be every bit as efficient as the 2.0t in the JLs and more efficient than the 3.6L. Since 2017 we've owned a 2017 ram rebel 5.7L hemi, a 2019 jlur 2.0t, a 2022 jlur with the xr package 3.6L, and a 2023 JT mojave 3.6L. The 2.0t was only slightly more efficient than the 5.7L stock for stock and the ram weighed about 1500lbs more. Adding armor, tires, and lift to the 2.0t and the efficiency dropped to 15-16mpg. The ram still got 17-18mpg after a lift and tires. The JLUR 3.6Lgets 13-14mpg as does the JTM. I would bet money that the JT would get better mileage from a 5.7L than the 2.0t as the efficiency of the 2.0 goes out the window when it's pushing boost constantly.

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