Need some advice on 37's.........

[roaniecowpony]

Hey everyone, im looking too maybe upgrade from 35s to 37s on my 21 wrangler high altitude 3.6, i have a 2.5 lift in it currently with the factory 20inch wheels and 1.5 inch spacers. Space wise it appears that the 37s will fit however I've seen some comments regarding the need for a regear to run the 37s and that the 3.6 would struggle. im not sure if thats accurate info or necessary too regear

? Any advice on this from anyone who os running a similar setup or has some experience with it.

Thanks everyone

Going up to 37s takes more of everything, which is code for more money and time. Steering will be your biggest area of concern. 37s will shimmy if your steering and suspension isn't as tight as it should be or your tires aren't balanced well or there's an out of round tire.

But, heck...

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[yokramer]

Going up to 37s takes more of everything, which is code for more money and time. Steering will be your biggest area of concern. 37s will shimmy if your steering and suspension isn't as tight as it should be or your tires aren't balanced well or there's an out of round tire.

But, heck...

just skip the 37s and go tons and 44s like all the youtubers.

 

[stretch-bsn]

I’m running small and light 37’s (BFG KO2’s) with 3.73 gears and a 3.6. I wheel it and daily drive it. What I notice…I make sure to watch the oil temp because I think it gets warmer than regular. I will regear. But I also do everything I want with it.

 

[WillysMeow]

I’m running small and light 37’s (BFG KO2’s) with 3.73 gears and a 3.6. I wheel it and daily drive it. What I notice…I make sure to watch the oil temp because I think it gets warmer than regular. I will regear. But I also do everything I want with it.

I forget, youb have Rubicon or the standard 2.73:1 transfer case?

@ShawnWhite78 Some good advice here, imo your axle gears are going to make it sluggish as heck.

 

[Beach Nut]

What combo is giving you 13 MPG? That's bizarre for anything that doesn't have a 392.

2020 Sahara 3.6 auto w/3.45 gears…awful gas mileage…oh, and that’s HIGHWAY mileage!

 

[stretch-bsn]

I forget, youb have Rubicon or the standard 2.73:1 transfer case?

@ShawnWhite78 Some good advice here, imo your axle gears are going to make it sluggish as heck.

I have a standard 2.73 transfer case. My axles are the 392 axles with the steel knuckles…got them off marketplace and put them on my sport.

 

[Maverick909]

100% regear. You are going to be slower than geo metro off the line daily acceleration minimum 4.88 axle ratio

 

[moabman-OG]

You will need bigger rims, probably 24" or 26" rims.

 

[AKLespaul]

I was just giving you a hard time about Ontario. Sorry it was missed.

 

[ShawnWhite78]

I’m running small and light 37’s (BFG KO2’s) with 3.73 gears and a 3.6. I wheel it and daily drive it. What I notice…I make sure to watch the oil temp because I think it gets warmer than regular. I will regear. But I also do everything I want with it.

Warmer oil temps while offroading? Or road driving or both? Thanks for the reply

 

[ShawnWhite78]

What’s your gear ratio now?​

Current factory gear ratio is 3.73

 

[rickinAZ]

I've used these 35x12.5x20 "knockoff" similar too toyo open country with load range E and have been fairly decent when you consider the discounted price . The ride hasn't been too bad . The E load range tho is likely an inaccurate rating. But I've been pleased with them . Ive had the mt and the at on different trucks over the years. Id be looking for the same brand in 37s if I go that route

In your case, the primary concern with E-range tires isn't just the ride quality or the price—it's the weight. While the extra 2 inches of height will naturally impact your gear ratio, the increased rotating mass is what really drags you down. If you're looking for a solution, consider the BFG KOs in C-range; they are consistently among the lightest 37s on the market. In fact if you trade your heavy 35" M/S Es for lighter 37" KO Cs, you may not feel much of a difference at all.

I actually had the chance to test this on my torquey EcoDiesel (running 3.73s) when Discount Tire accidentally mounted E-range KOs instead of the C-range I ordered. Swapping them back-to-back, the difference was night and day. With the E-range tires, it felt like I was hauling a 250lb passenger, even with an engine that typically handles 3.73 gears without breaking a sweat.

However, I don't think you can achieve that balance without moving away from the 20-inch wheels. Keeping the 20s severely limits your tire options. Since your focus is on street driving and achieving a specific aesthetic, 17-inch wheels will actually provide a much better look for this setup. I know that might not be the news you were hoping for, but the consensus here is that 20-inch wheels simply involve too many compromises for what you're trying to do.

 

[stretch-bsn]

Warmer oil temps while offroading? Or road driving or both? Thanks for the reply

So I had falkin wildpeaks in a 315x12.5x17 and they were around 73lbs per tire. I overheated on a cross country trip to Moab with those (oil temps got to around 250+ and I pulled over). No issues after that however, I became paranoid after that experience.

I went with the KO2’s because they are c rated and weigh roughly 64lbs per tire. So 9 lbs drop in weight per corner (even though rolling mass/bigger tire is more).

Since overheating that one time with my 315’s I’ve always watched my oil temps. The temps I’m getting are both on road and off roading. My oil temps will creep up to 217/219 and I’ll hit my AC for my fan to turn on and bring the temps down. I know the 3.6 fan kicks on by itself at 226 but I don’t want to get into a spot where my motor is just a constant 226 (I don’t have data to back it up but I’m guessing one of the reasons these 3.6’s fail is because they run hot and people just assume the factory settings are fine which in turn wears out motor components faster).

The reason I say runs warmer is I’ve run with a buddy who has a 2020 rubicon gladiator on 35’s MT’s (don’t remember brand). While driving down the highway, backroads, and wheeling, I’ve asked him his oil temps and they were pretty consistently 10 degrees Fahrenheit lower than mine in all conditions. My Dad has a 2 door rubicon (2.0 motor, 33” tires) and I had the same experience (10 degrees). So that is why I say it runs warmer. I negate this by being hawkish about monitoring my oil temps and prematurely kicking on the fan instead of letting the computer do it.

I know that was long winded but hopefully it helped.

 

[stretch-bsn]

In your case, the primary concern with E-range tires isn't just the ride quality or the price—it's the weight. While the extra 2 inches of height will naturally impact your gear ratio, the increased rotating mass is what really drags you down. If you're looking for a solution, consider the BFG KOs in C-range; they are consistently among the lightest 37s on the market. In fact if you trade your heavy 35" M/S Es for lighter 37" KO Cs, you may not feel much of a difference at all.

I actually had the chance to test this on my torquey EcoDiesel (running 3.73s) when Discount Tire accidentally mounted E-range KOs instead of the C-range I ordered. Swapping them back-to-back, the difference was night and day. With the E-range tires, it felt like I was hauling a 250lb passenger, even with an engine that typically handles 3.73 gears without breaking a sweat.

However, I don't think you can achieve that balance without moving away from the 20-inch wheels. Keeping the 20s severely limits your tire options. Since your focus is on street driving and achieving a specific aesthetic, 17-inch wheels will actually provide a much better look for this setup. I know that might not be the news you were hoping for, but the consensus here is that 20-inch wheels simply involve too many compromises for what you're trying to do.

I agree with this right here…spot on. Even when I went from 315’s e rated to 37’s c rated (ko2) the difference was night and day. Such a better ride quality and I’m still getting between 17.5-18.5 mpg….but I do drive like an old man when I’m not off roading lol

 

[roaniecowpony]

So I had falkin wildpeaks in a 315x12.5x17 and they were around 73lbs per tire. I overheated on a cross country trip to Moab with those (oil temps got to around 250+ and I pulled over). No issues after that however, I became paranoid after that experience.

I went with the KO2’s because they are c rated and weigh roughly 64lbs per tire. So 9 lbs drop in weight per corner (even though rolling mass/bigger tire is more).

Since overheating that one time with my 315’s I’ve always watched my oil temps. The temps I’m getting are both on road and off roading. My oil temps will creep up to 217/219 and I’ll hit my AC for my fan to turn on and bring the temps down. I know the 3.6 fan kicks on by itself at 226 but I don’t want to get into a spot where my motor is just a constant 226 (I don’t have data to back it up but I’m guessing one of the reasons these 3.6’s fail is because they run hot and people just assume the factory settings are fine which in turn wears out motor components faster).

The reason I say runs warmer is I’ve run with a buddy who has a 2020 rubicon gladiator on 35’s MT’s (don’t remember brand). While driving down the highway, backroads, and wheeling, I’ve asked him his oil temps and they were pretty consistently 10 degrees Fahrenheit lower than mine in all conditions. My Dad has a 2 door rubicon (2.0 motor, 33” tires) and I had the same experience (10 degrees). So that is why I say it runs warmer. I negate this by being hawkish about monitoring my oil temps and prematurely kicking on the fan instead of letting the computer do it.

I know that was long winded but hopefully it helped.

There are some modifications you can do to lower your oil and coolant temperatures.

The most effective thing is changing the fan schedule to come on at lower temperatures. RPM Extreme sells a fan control module that you splice into your PCM harness. It's very effective. The other method equally effective, is reprogramming the PCM fan schedule with HPTuner , Diablo etc. Other mods can help reprogramming to be more effective in more extreme conditions. Larger radiators, such as Mishimoto, add more margin. A higher output 850w or 940w, fan from Gladiators ,392s,etc are also effective for adding margin.

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