37" Tires & Gearing

[Brew211]

First let me say this forum is incredible! While I wait for delivery it allows me to create my future build and learn along the way.

My question is this, I plan on 37" tires (don't know 12.5 or 13.5) and I've read some threads that say gearing isn't necessary and others that say its a must. I"m looking for some solid advice on whether or not I need to because I really don't want to deal with that hassle and may need to settle for 35's. I've read several of you have already installed 37's so I'm hoping you can help.

- Manual transmission
- Rubicon
- Live in Arizona
- This is a second vehicle so not my primary driver
- Will be using off road and will also be driving on the highway to work occasionally

Thank you so much for your help.

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[Hailchaser]

I just finished a on and off-road trip through Colorado. Mine is an auto RU with 37’s. I’m definitely going to regear, likely 4.56. RPM’s run too high.

 

[Brew211]

Brew211,
My situation is similar to yours; JLR, stick shift, 37s, but I live at 6000 feet, which means I have 15%-20% less horsepower than you. First gear is plenty low, reverse is just a little high. Fifth and sixth gears are really tall, but usable, particularly on flat terrain, even up to higher speeds. In fact, if you get the RPM above 2000 (this must be where the cam timing changes) it will pull sixth quite nicely. You should know I'm coming from a stick shift TJ with 33s and it couldn't get off a piece of bubble gum without a winch.

Like you, I thought about re-gearing, but it's simply not necessary. Nice, maybe, but not necessary. In fact, I ran an online gearing calculator and the results were surprising; 4.10 axles with 37s is the exact same final drive ratio as the sport model's 3.40-something axles with stock tires.

I hope this helps.

Randy

Thank you very much for taking the time to write. This is encouraging to go for the 37's if its working for you at that elevation. I live in Arizona and while the majority of the time I'll be at 2,500, I regularly drive up north which can goes from 4,500-7,000. Thank you again.

Thank you Hailchaser too.

 

[Brew211]

Like you, I waffled between 35s and 37s, my prior TJ had only 33s and I was fearful 37s would be too large. I decided on 37s, but didn't even drive the Jeep before installing them. My daughter pulled the new Jeep into the shop and I installed a cheap spacer lift with shock extensions, an adjustable front track bar, and the 37s.
As tall as the gearing with 37s seems, I'm real glad I didn't settle for 35s.
You would enjoy having the TazerJL too, in part for the tire compensation, but also for the other features. If you get one, you'll need to program the actual bottom-side tire diameter, not the marketing-size designation (37). I believe mine is set for 36.4 and the speedo is spot-on.

Thank you again for the time and the advice. I have to look into this TazerJL thing, I keep hearing about it but don't know anything about it.

 

[tobyw]

I just completed a 2,700 mile round trip from Washington state down to the Mexico border (and ran the Rubicon Trail along the way ) with our JLU-R automatic on 37's. We ran a mix of interstate and scenic byways, everything from sea level to nearly 9,000ft, and I can honestly say the Jeep doesn't need gears unless you are extremely annoyed with transmission hunting. With that said, the nature of the 8-speed platform lends itself to transmission hunting anyways in order to keep the engine in it's powerband, so even on the stock 33's I would see some hunting on the freeway around here in the PNW. The Jeep has plenty of power, even for passing maneuvers at speed and elevation, so long as you let the engine rev a little and let the transmission do it's thing. I have no intentions of re-gearing mine.

 

[Brew211]

I just completed a 2,700 mile round trip from Washington state down to the Mexico border (and ran the Rubicon Trail along the way ) with our JLU-R automatic on 37's. We ran a mix of interstate and scenic byways, everything from sea level to nearly 9,000ft, and I can honestly say the Jeep doesn't need gears unless you are extremely annoyed with transmission hunting. With that said, the nature of the 8-speed platform lends itself to transmission hunting anyways in order to keep the engine in it's powerband, so even on the stock 33's I would see some hunting on the freeway around here in the PNW. The Jeep has plenty of power, even for passing maneuvers at speed and elevation, so long as you let the engine rev a little and let the transmission do it's thing. I have no intentions of re-gearing mine.

Well done!!! Thank you so much for sharing.

 

[NFRs2000NYC]

Id rather have the strength vs slightly better gearing. Smaller pinion means weaker...and with the 8 speed its just not that necessary...from 4.10s that is.

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