For those going with 37s, are you regearing??

CarbonSteel

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I re-geared with 35's and am definitely glad I did especially since I am now running 37's. I do not understand those who say 4.10's are "just fine" with 35's and 37's. Speaking from experience, if putting it in gear and it moving means it is "just fine", then OK, but there is no comparison between 4.10's and 4.88's with a 3.6L and an 8 speed.

Mine would not hold 8th and sometimes not even 7th, constantly up and down shifted, and MPG suffered. That is all history after the re-gear.

YMMV!
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2.0t, pretty heavy geolander x-mt's, steel bumpers, winch, hi-lift, 200+ lbs worth of tools/compressor/recovery gear in the Jeep at all times. If you kept the weight down I think the 3.6L would be fine. The 8 speed does a great job of keeping the engine in it's power band. I think people make way to big of a deal about 8th gear anyway. If you're spending more time in 8th gear because you are always at a higher rpm, are you really gaining anything? Even 7th gear is still an overdrive, and I'm far more concerned with the rpm range cruising at freeway speed than the gear the transmission is in. Even if you only see 8th on downhills like some claim, you are still at lower rpms in those scenarios than the guys with 4.88s will ever see at speed.
This is what I'm thinking. We're all pushing these refrigerators, so we can't be in a rush. I would still like to hit 80 on the freeway though w/o regearing. Hopefully a 3.6l can do so flat if not completely loaded down (w 37s).
 

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I re-geared with 35's and am definitely glad I did especially since I am now running 37's. I do not understand those who say 4.10's are "just fine" with 35's and 37's. Speaking from experience, if putting it in gear and it moving means it is "just fine", then OK, but there is no comparison between 4.10's and 4.88's with a 3.6L and an 8 speed.

Mine would not hold 8th and sometimes not even 7th, constantly up and down shifted, and MPG suffered. That is all history after the re-gear.

YMMV!
It could be that the 3.6L is happier at higher rpms, but the idea that we want our 8 speed transmission to "hold" 8th gear and never shift is lost on me. That tells me I'm geared too low as my jeep is always at a high enough rpm to never downshift? My 2.0t and 8 speed seem very happy around town, pulling grades in the high Sierra's, off road, and cruising at 85mph for 8+ hours at a time heading out to Arizona, Utah, or Colorado to go wheeling. Sure 4.56 would be right back at factory gearing (technically 4.59 if we are being exact), but I doubt FCA engineered the Jeep for idiots like me who want to cruise 85 anyway. Even with all the heavy mods, the extra 2-300lbs worth of tools/compressor/recovery gear in back, and 37's she will hit 0-60 in 6.88s according to the tazer, which is far better than "putting it in gear and it moving" or "just fine" as far as I'm concerned. 17 mpg on 37's for someone who drives on the aggressive side is great in my opinion. My ram averages 12-13 with the 5.7L on 35's, making the Jeep our economy car.
 

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It could be that the 3.6L is happier at higher rpms, but the idea that we want our 8 speed transmission to "hold" 8th gear and never shift is lost on me. That tells me I'm geared too low as my jeep is always at a high enough rpm to never downshift? My 2.0t and 8 speed seem very happy around town, pulling grades in the high Sierra's, off road, and cruising at 85mph for 8+ hours at a time heading out to Arizona, Utah, or Colorado to go wheeling. Sure 4.56 would be right back at factory gearing (technically 4.59 if we are being exact), but I doubt FCA engineered the Jeep for idiots like me who want to cruise 85 anyway. Even with all the heavy mods, the extra 2-300lbs worth of tools/compressor/recovery gear in back, and 37's she will hit 0-60 in 6.88s according to the tazer, which is far better than "putting it in gear and it moving" or "just fine" as far as I'm concerned. 17 mpg on 37's for someone who drives on the aggressive side is great in my opinion. My ram averages 12-13 with the 5.7L on 35's, making the Jeep our economy car.
LOL - my Jeep will downshift when it needs to or makes sense to--it is not "stuck" in 8th gear at highway speeds. What it does not do is stay in 6th or 7th at highway speeds nor does it constantly up and downshift like it did with 4.10s.

I am definitely not overgeared and I can run 85+ MPH all day if I wanted to, but I tend to run 75-80 in 75MPH speed zones. My RPMs are 150-250 higher than with 4.10 gears and that is no big deal. I also manage 16-17MPG at 75 and 18-20 at 65 or lower.

A 3.6L + 8 speed + 4.10 gears + 35" or 37" tires != A good driving experience.
 
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Original tires were 33's and I went with 39's and I said similar, not the same... Simple math dictates this fact; in order to cover the same amount of ground, a small tire must rotate more times than a larger tire. Because of this, the taller the tire, the faster it propels the vehicle, even as the vehicle's engine produces the same number of rotations per minute (RPM). My RPM's are very close with the gear set up and increased tire size take a look at the calculator below from the Spicer page. The JL has 8 gears so the Overdrive is the number that would be closest. Not sure where you are getting your info from...
Stock.PNG

39.PNG
Sorry, newbie here, am I correct in assuming my stock 4.10 ratio on 35's at 2200 rpm is faster in 5th/6th gears than if I re-geared to 4.56 with same rpms and tire size? Thus, slower start/less power from a stop but better highway mileage?
Would the gas mileage suffer at slower speeds/stop/starting with 4.10 instead of 4.56?
 

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If you have a 3.6, re gear, 2.0T it's optional. I am super heavy, 37s with real beadlocks, aluminum skid plates, RTT, full payload etc. With that, not only do I get all 8 gears here in Colorado, I have hit 112mph to pass some semis. I just recently bought a [email protected] 400 BD Camper trailer and still, 4.10s, I have no problem towing 55mph. I admit, it could be better at towing, and I have a set of 40s I like to throw on during trail season, so I'm jumping to 5.13s.
 

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Sorry, newbie here, am I correct in assuming my stock 4.10 ratio on 35's at 2200 rpm is faster in 5th/6th gears than if I re-geared to 4.56 with same rpms and tire size? Thus, slower start/less power from a stop but better highway mileage?
Would the gas mileage suffer at slower speeds/stop/starting with 4.10 instead of 4.56?
You actually start quicker with lower gears. The mileage generally suffers at the top speed with lower gears. You can use the Spicer gear chart (that has been around forever and was made by a gear company by engineers) to get pretty close to the RPM's or you can use the grimmreaper chart that was made by a guy on the internet and get a very similar number. Both charts have disclaimers that they could or could not be accurate for your application.
 
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Zandcwhite

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You actually start quicker with lower gears. The mileage generally suffers at the top speed with lower gears. You can use the Spicer gear chart (that has been around forever and was made by a gear company by engineers) to get pretty close to the RPM's or you can use the grimmreaper chart that was made by a guy on the internet and get a very similar number. Both charts have disclaimers that they could or could not be accurate for your application.
One is a generic estimate that assumes 3rd gear ratio is 1-1 (none of us run 3 speeds as it's not the cj forum but a jl forum therefore that would be your 6th gear rpm), assumes your Overdrive ratio(which is actually closer to our 7th gear as our 8 speed has 2 Overdrive gears), and has been around forever with old information. The other is updated for all types of drivetrain changes, including our 8 speed transmission. If you input the actual measured diameter of your tires on your rig it will be 100% accurate every time as your rpms are simple math. But it was "made by a guy on the internet" which would only make those with the inability to understand and verify the accuracy of the math nervous.
 

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I had a 2018 JLUR with 37's I did not re-gear. I assure you it NEEDED it though. If you drive in the city all the time you will notice nothing it drives great. It is a highway issue. You will 100% lose 8th gear and 7th is only useable with the transmission in manual mode.

Is it driveable, absolutely. Is it just fine or good as ever, 100% NO. People need to quit posting that it is fine and you do not need to. It gives folks a false understanding of the situation created when adding 37's. Can you get away with not re-gearing, sure. That does not make it right as rain and all systems good.

Another thing I see over and over is folks saying 4.56 is the money on 37's. 4.56 gets you back in the neighborhood gearwise as the 4.10's. It in no way addresses the added mass of the giant tires and typically heavier wheels. More importantly it does not address the drastic change in drag your Jeep sees once lifted 3.5" or more. 4.88's is what I think the minimum would be to get you into an operating range with some power.

Another thing worth considering is the 3.6 is a 24v high winding engine with a fairly poor torque curve at lower RPM's. Gearing to turn 2500-2900 at 70 MPH is what will make the difference on having some power. The engine has a 6500 RPM redline.

If you are primarily concerned with MPG you should not be having a 37" tire conversation in the first place.

I just took delivery on a 2021 JLUR with 6spd manual. I am about to add a Clayton lift and 37's. I have the 5.13 gear sets already sitting in my garage for the regear. It will run fantastic in town or the highway once done.

Just my thoughts.
 

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I had a 2018 JLUR with 37's I did not re-gear. I assure you it NEEDED it though. If you drive in the city all the time you will notice nothing it drives great. It is a highway issue. You will 100% lose 8th gear and 7th is only useable with the transmission in manual mode.

Is it driveable, absolutely. Is it just fine or good as ever, 100% NO. People need to quit posting that it is fine and you do not need to. It gives folks a false understanding of the situation created when adding 37's. Can you get away with not re-gearing, sure. That does not make it right as rain and all systems good.

Another thing I see over and over is folks saying 4.56 is the money on 37's. 4.56 gets you back in the neighborhood gearwise as the 4.10's. It in no way addresses the added mass of the giant tires and typically heavier wheels. More importantly it does not address the drastic change in drag your Jeep sees once lifted 3.5" or more. 4.88's is what I think the minimum would be to get you into an operating range with some power.

Another thing worth considering is the 3.6 is a 24v high winding engine with a fairly poor torque curve at lower RPM's. Gearing to turn 2500-2900 at 70 MPH is what will make the difference on having some power. The engine has a 6500 RPM redline.

If you are primarily concerned with MPG you should not be having a 37" tire conversation in the first place.

I just took delivery on a 2021 JLUR with 6spd manual. I am about to add a Clayton lift and 37's. I have the 5.13 gear sets already sitting in my garage for the regear. It will run fantastic in town or the highway once done.

Just my thoughts.
This is the best write-up on this thread.......I have a 20 JLUR, 3.6 Auto, on 37's, Beadlocks and usual add-on weight (bumpers/winch, etc). The BEST mod that I've made is to re-gear. Went to 4:88's - if my wheeling was more aggressive or in the mountains I would have gone to 5:13. I have to drive a distance to wheel so on the highway when running the old 4.10's my 8th gear was 98% lost (was OK downhill with a backwind haha), and driving on highways fluctuated between 6th and 7th. Around town on 4.10's it was drivable, BUT going to 4:88's placed me back in the powerband and put a smile back on my face. (Mileage...I'm on 37's so don't do it if you're trying to save pennies...If you see the reason to go this route expect somewhere around 15-17mpg and enjoy the experience..!!)
 

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This is the best write-up on this thread.......I have a 20 JLUR, 3.6 Auto, on 37's, Beadlocks and usual add-on weight (bumpers/winch, etc). The BEST mod that I've made is to re-gear. Went to 4:88's - if my wheeling was more aggressive or in the mountains I would have gone to 5:13. I have to drive a distance to wheel so on the highway when running the old 4.10's my 8th gear was 98% lost (was OK downhill with a backwind haha), and driving on highways fluctuated between 6th and 7th. Around town on 4.10's it was drivable, BUT going to 4:88's placed me back in the powerband and put a smile back on my face. (Mileage...I'm on 37's so don't do it if you're trying to save pennies...If you see the reason to go this route expect somewhere around 15-17mpg and enjoy the experience..!!)
Second best post in this thread and I fully agree with you and @rlong69 on all points. Does it HAVE to be re-geared? No, but you are kidding yourself if you think that adding larger, heavier tires and potentially wheels is not going to change the overall performance of the vehicle. I found out by experience of driving with 4.10s and then with 4.88s. There is no comparison.
 

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One is a generic estimate that assumes 3rd gear ratio is 1-1 (none of us run 3 speeds as it's not the cj forum but a jl forum therefore that would be your 6th gear rpm), assumes your Overdrive ratio(which is actually closer to our 7th gear as our 8 speed has 2 Overdrive gears), and has been around forever with old information. The other is updated for all types of drivetrain changes, including our 8 speed transmission. If you input the actual measured diameter of your tires on your rig it will be 100% accurate every time as your rpms are simple math. But it was "made by a guy on the internet" which would only make those with the inability to understand and verify the accuracy of the math nervous.
Even his page says that it is not always accurate. Figure it out on paper if you need to. I did prior to installing the gears myself and came out just about spot on with getting back to the stock RPM's with the 5.13 and 39's. If you use the overdrive number the result is still similar regardless of which calculator you use. Which is the opposite of what you said in your post asking me which planet I came from. By the way, how many sets of gears and lockers have you personally set and installed on your rigs?
 

Zandcwhite

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Even his page says that it is not always accurate. Figure it out on paper if you need to. I did prior to installing the gears myself and came out just about spot on with getting back to the stock RPM's with the 5.13 and 39's. If you use the overdrive number the result is still similar regardless of which calculator you use. Which is the opposite of what you said in your post asking me which planet I came from. By the way, how many sets of gears and lockers have you personally set and installed on your rigs?
The only way the math for rpms is inaccurate is if you input your information incorrectly. A given diameter wheel will have a given number of rotations at a given speed 100% of the time. If you input the correct transmission, the correct axle gears, and most importantly the correct mounted diameter of YOUR tires as that varies greatly there is no margin of error. The math never lies. It’s not an estimate. Feed in accurate information and the output will always be accurate. It is especially problematic when you input tire sidewall diameter on something like a KO2. The measured height of brand new 37” KO2’s mounted on my buddies gladiator was 35.5”. Input the sidewall diameter and the calculator looks like it’s way off. Input 35.5” and there is no margin of error in any gear at any speed. That’s just not how it works. There’s no magic, just basic math. Short of a slipping transmission, it Is physically impossible for wheel speed to be different than engine speed x gearing. That’s literally how the speedometer works, wheel rpm input by the wheel sensor calculated using the tire diameter input into the computer for Christ sake. That’s why your speedo is of if you don’t input the correct tire size.
 

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I’m actually impressed with how the 3.6 paired with the 8 speed does with 37’s. I’ll probably regear eventually, but it’s moved to the bottom of my list.
 

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I had a 2018 JLUR with 37's I did not re-gear. I assure you it NEEDED it though. If you drive in the city all the time you will notice nothing it drives great. It is a highway issue. You will 100% lose 8th gear and 7th is only useable with the transmission in manual mode.

Is it driveable, absolutely. Is it just fine or good as ever, 100% NO. People need to quit posting that it is fine and you do not need to. It gives folks a false understanding of the situation created when adding 37's. Can you get away with not re-gearing, sure. That does not make it right as rain and all systems good.

Another thing I see over and over is folks saying 4.56 is the money on 37's. 4.56 gets you back in the neighborhood gearwise as the 4.10's. It in no way addresses the added mass of the giant tires and typically heavier wheels. More importantly it does not address the drastic change in drag your Jeep sees once lifted 3.5" or more. 4.88's is what I think the minimum would be to get you into an operating range with some power.

Another thing worth considering is the 3.6 is a 24v high winding engine with a fairly poor torque curve at lower RPM's. Gearing to turn 2500-2900 at 70 MPH is what will make the difference on having some power. The engine has a 6500 RPM redline.

If you are primarily concerned with MPG you should not be having a 37" tire conversation in the first place.

I just took delivery on a 2021 JLUR with 6spd manual. I am about to add a Clayton lift and 37's. I have the 5.13 gear sets already sitting in my garage for the regear. It will run fantastic in town or the highway once done.

Just my thoughts.
This is the best write-up on this thread.......I have a 20 JLUR, 3.6 Auto, on 37's, Beadlocks and usual add-on weight (bumpers/winch, etc). The BEST mod that I've made is to re-gear. Went to 4:88's - if my wheeling was more aggressive or in the mountains I would have gone to 5:13. I have to drive a distance to wheel so on the highway when running the old 4.10's my 8th gear was 98% lost (was OK downhill with a backwind haha), and driving on highways fluctuated between 6th and 7th. Around town on 4.10's it was drivable, BUT going to 4:88's placed me back in the powerband and put a smile back on my face. (Mileage...I'm on 37's so don't do it if you're trying to save pennies...If you see the reason to go this route expect somewhere around 15-17mpg and enjoy the experience..!!)

Pretty much this. I don't live in the mountains, so I have gotten by quite well, to where I could say...go for it. But, I am still looking strongly at going to 4.88s since I just got beadlocks...more weight, and plan for a small trailer, more weight.

I am agreeing and I see 8th "naturally" on 37s with 4.10s. I don't see it for long periods. I had my JLUR for weeks before 37s so I don't even know what stock really should be.
 
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