JLU Rubicon Auto: 35" and 37" - Regearing Math

Starting at what tire size (on the JLUR) would you regear?


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Halstem1

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Any suggestions on re-gearing and pulling a small trailer? Would it be beneficial to gear lower to accommodate the extra load? For example, if I was planning on going to 4.88's and then purchased a 2500# trailer, would 5.13 be a better decision? Or should I not base my decision off of the trailer since the jeep is my daily driver?

Concerns:
1) Gearing to 4.88 and still not able to use 7th and 8th on highway when pulling trailer.
2) Gearing to 5.13 and running above 2500 rpms on the daily or even higher rpms on road trips.

What is a comfortable highway RPM? What does 37's and 5.13's run for rpms doing 80 mph?





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Unkle Fester

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my high school Algebra Teacher was right, one day math will save the world.

i loved the detail and the thought put into the OP. But I think it comes down to what you are trying to achieve. The original gearing equation is nominal. all three engines use either 3:73 or 4:10 (towing package) regardless of transmission. I think it’s a ratio range and within that range, you can decide where you feel comfortable. Stress on the system at a standing stop acceleration (anywhere) and performance in Overdrive (highway) play a factor. Low range especially in a Rubicon is geared down about 4:1 (I think).

i run the stock 33’s and am stepping up to 35’s next week. With my 4:10 gearing I’m still in the good range for fuel economy. My 4 low ratio more than compensates. Here is a chart from a popular jeep parts website...
9831DDDA-2C00-40D3-B05E-6547080D93B7.jpeg

yellow is best fuel economy/highway Performance
green is best overall (daily driver)
blue is best torque, (towing) lowest economy.

Stock, a Rubicon with the tow package is in the bottom of the green, not in the center, which would be ideal in a perfect world. I think this is to give better fuel economy. but it’s a range of what you are trying to achieve. 35’s will put me in the middle of the yellow. In 4 low, this is mitigated by the transfer case and I save the cost of re-gearing.

keep this in mind, at some point taller gears means a new carrier, and that can get real spendy.

just trying to take the guesswork (and the extra math) out of gear ratios.
 

Unkle Fester

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Any suggestions on re-gearing and pulling a small trailer? Would it be beneficial to gear lower to accommodate the extra load? For example, if I was planning on going to 4.88's and then purchased a 2500# trailer, would 5.13 be a better decision? Or should I not base my decision off of the trailer since the jeep is my daily driver?

Concerns:
1) Gearing to 4.88 and still not able to use 7th and 8th on highway when pulling trailer.
2) Gearing to 5.13 and running above 2500 rpms on the daily or even higher rpms on road trips.

What is a comfortable highway RPM? What does 37's and 5.13's run for rpms doing 80 mph?
When towing, you don’t want overdrive to engage regardless of gear range... Thats why trucks have a tow mode to disengage overdrive.
 

BHo

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my high school Algebra Teacher was right, one day math will save the world.

i loved the detail and the thought put into the OP. But I think it comes down to what you are trying to achieve. The original gearing equation is nominal. all three engines use either 3:73 or 4:10 (towing package) regardless of transmission. I think it’s a ratio range and within that range, you can decide where you feel comfortable. Stress on the system at a standing stop acceleration (anywhere) and performance in Overdrive (highway) play a factor. Low range especially in a Rubicon is geared down about 4:1 (I think).

i run the stock 33’s and am stepping up to 35’s next week. With my 4:10 gearing I’m still in the good range for fuel economy. My 4 low ratio more than compensates. Here is a chart from a popular jeep parts website...
9831DDDA-2C00-40D3-B05E-6547080D93B7.jpeg

yellow is best fuel economy/highway Performance
green is best overall (daily driver)
blue is best torque, (towing) lowest economy.

Stock, a Rubicon with the tow package is in the bottom of the green, not in the center, which would be ideal in a perfect world. I think this is to give better fuel economy. but it’s a range of what you are trying to achieve. 35’s will put me in the middle of the yellow. In 4 low, this is mitigated by the transfer case and I save the cost of re-gearing.

keep this in mind, at some point taller gears means a new carrier, and that can get real spendy.

just trying to take the guesswork (and the extra math) out of gear ratios.
What transmission gear ratio is your (math) chart based off of? Seems to be missing from most folks comments.
 

ChattVol

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my high school Algebra Teacher was right, one day math will save the world.

i loved the detail and the thought put into the OP. But I think it comes down to what you are trying to achieve. The original gearing equation is nominal. all three engines use either 3:73 or 4:10 (towing package) regardless of transmission. I think it’s a ratio range and within that range, you can decide where you feel comfortable. Stress on the system at a standing stop acceleration (anywhere) and performance in Overdrive (highway) play a factor. Low range especially in a Rubicon is geared down about 4:1 (I think).

i run the stock 33’s and am stepping up to 35’s next week. With my 4:10 gearing I’m still in the good range for fuel economy. My 4 low ratio more than compensates. Here is a chart from a popular jeep parts website...
9831DDDA-2C00-40D3-B05E-6547080D93B7.jpeg

yellow is best fuel economy/highway Performance
green is best overall (daily driver)
blue is best torque, (towing) lowest economy.

Stock, a Rubicon with the tow package is in the bottom of the green, not in the center, which would be ideal in a perfect world. I think this is to give better fuel economy. but it’s a range of what you are trying to achieve. 35’s will put me in the middle of the yellow. In 4 low, this is mitigated by the transfer case and I save the cost of re-gearing.

keep this in mind, at some point taller gears means a new carrier, and that can get real spendy.

just trying to take the guesswork (and the extra math) out of gear ratios.
The gear chart you posted isnt remotely accurate for a JL. You should consider deleting it off here to not mislead folks.
 

BHo

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conFUcius

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Stupid question but it was not quite clear... should I be doing calculations off of the actual tire measurement or the paper specs? I entered 35” for my 37/12.5/17 KO2s at 32 PSI to recalibrate the speedometer accurately. While the 4.10 is manageable, I’ve certainly lost 8th gear any time I hit any slight incline trying to drive 75 to 80 MPH. Leaning towards the 5.13s when it comes time but would that be over-geared?

I have totally different goals in mind than your average owner. My goal is to create a sleeper street racer! I know this sounds ludicrous but it’s my choice. I have a 2018 JL (2 door). I upgraded to 315/70 R17 BFG K02’s with a Mooar 2” lift. Here’s where I depart from almost all of you. I have just completed installation of Prodigy Performance Stage 2 turbo kit. With th3 stock 3.45 ratio I am not happy wirh the 0-60 acceleration. I don’t care about gas mileage but I don’t want to damage the drive train. I have the 8 speed auto on a Sport S. I am considering the 4,88 ratio. Any suggestions.
Same here, I can’t wait to drift in my Fast and Furious Street Jeep :LOL:. What did you end up going with?
 

Rodeoflyer

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You can't 'over' gear. You can have a non-ideal gearing 'issue' but you can't 'overgear'. Well no, you could, but that gearing isn't readily available for civilian use lol.

Our transmissions (both auto and manual) have two overdrives and we're using heavy, offroad tires at 34.5+ diameter with heavy aftermarket wheels.
 

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