Hard data on 37s

Jeepherz · Oct 31, 2023

I just found a great deal on new 37" BFG KO2 37/12.5/17s. $1000 for (4) new tires. My JK needs new tires badly so I am considering putting my Recon 35s on my JK; and throwing the 37s on my JL.
Question- has anyone actually clocked actual performance numbers with a swap to 37s? RPMs? 0-60 etc? On a Recon 392/4.56s? I have no other mods. I am a horsepower girl and I bought the 392 for a reason. I know to expect a change but I am looking for exactly how much? Back in the day there was a chart for the JKs showing RPMs turning at each gear ratio/by tire size. I can find no hard data aside from seat-of the pants feel opinions. I've been on a 1000 JK gear/tire threads by now and have read too many things like "3.21s and 35s are fine" and "its a Jeep so its ok to be slow" etc. so I am a little cautious on going by opinions.,
(I did search the forum because I know there have been many 37s threads but I get an error when I search "37s". TIA.)

Viking Jeeper · Oct 31, 2023

No data to provide but you will notice a difference. I certainly did when moving up from 33 to 35 with 3.73. I installed a pedal commander and it all came back. Lately the PC has had a few failures and many people have moved onto Banks Pedal Monster. I'm still running the PC with no issues. If you want rpm data you can add all of your specs at www.grimmjeeper.com

Oh and I would JUMP on 4 of those for $1k.

Jeepherz · Oct 31, 2023

Viking Jeeper said:
No data to provide but you will notice a difference. I certainly did when moving up from 33 to 35 with 3.73. I installed a pedal commander and it all came back. Lately the PC has had a few failures and many people have moved onto Banks Pedal Monster. I'm still running the PC with no issues. If you want rpm data you can add all of your specs at www.grimmjeeper.com

Oh and I would JUMP on 4 of those for $1k.

Thanks. I forgot about Grimm's calculator.
THis is what I have:

ZF8HP75 (tranny)
MP3022(Transfer case)
Atlas 4 speed box

Is this correct?
Yah- that price is making it hard for me to come to my senses. I am not off-roading the 392 but it looks so puny next to my baby JK with a 4" lift.
Not only am I driving a 4 door with a hard top- stock- I want to throw on bigger tires for looks.
What has become of me?

prodriquez · Oct 31, 2023

The 4:56 was made to run 37" tires. I don't have any hard numbers for you... But my wife took my 37" Cooper STT Pros when I moved to 1-tons and 40's. Under acceleration, it feels like we added an extra passenger, which isn't much of an impact.

biochem · Oct 31, 2023

I found a great deal on these exact tires as well and the 37s went on my 392 XR. I also installed a Clayton 3.5 inch lift. Tires are on KMC tank beadlocks.

I don’t have an official 0-60 number but the 392 still feels plenty fast and will still put you back in the seat. It doesn’t feel like it has lost much with the bigger tires.

One thing to note is stock 35’s are actually only between 34-35 inches in actual diameter. 37’s are actually only about 35.5 ish in diameter (I don’t have the exact measurements I took on me).

So really, it isn’t as big of a difference as you would expect which, in my opinion, is actually a good thing. Little beefier tire without a significant drop in performance.

Viking Jeeper · Nov 1, 2023

BF Goodrich says the 37 inch tires are exactly 2 inches bigger than the 315/70-17.

BenLandry · Nov 1, 2023

I’m still yet to be beat from Red light to red light with 37s and 4:56 gears….real world performance measurement. Recently raced a Challenger with a Hemi badge, probably 5.7. Beat him at 2 red lights to 100mph by a full car length. I let off at 100 and he came by me. He got the picture tho.

biochem · Nov 1, 2023

Viking Jeeper said:
BF Goodrich says the 37 inch tires are exactly 2 inches bigger than the 315/70-17.

Maybe at lab specs at a certain tire pressure but not in the real world. I have both sets and have measured them in pressures ranging from 30-35 psi.

Viking Jeeper · Nov 1, 2023

biochem said:
Maybe at lab specs at a certain tire pressure but not in the real world. I have both sets and have measured them in pressures ranging from 30-35 psi.

and what were the results?

TRMFAM · Nov 1, 2023

Viking Jeeper said:
and what were the results?

Post #5 stated:

One thing to note is stock 35’s are actually only between 34-35 inches in actual diameter. 37’s are actually only about 35.5 ish in diameter (I don’t have the exact measurements I took on me).

Viking Jeeper · Nov 1, 2023

TRMFAM said:
Post #5 stated:

(I don’t have the exact measurements I took on me).

Post #5 said the numbers given were not exact. If we are discussing the difference between 1.5 and 2.0 inches I think its fair to get exact numbers.

Jeepherz · Nov 1, 2023

Size matters.

prodriquez said:
The 4:56 was made to run 37" tires. I don't have any hard numbers for you... But my wife took my 37" Cooper STT Pros when I moved to 1-tons and 40's. Under acceleration, it feels like we added an extra passenger, which isn't much of an impact.

I'd honestly feel better if they were 4.88s. I have 4.56s and 35s on my much lighter JK. Good combo but most people that I know on 37s/4 doors went to 4.88s. But those were also non-392s. That's where I am lacking data. Trying to compare the difference on a Pentastar vs. the 392 just isn't remotely the same. I don't notice much of a difference when I've got passengers so....

Viking Jeeper · Nov 1, 2023

Do you have PC or pedal monster? The first run of 392's with 3.73 and 33's are rockets. 37's with 4.56 is even lower gearing. I'm willing to bet you can go 37's and add a pedal upgrade and be faster than right now.

Jeepherz · Nov 1, 2023

I've generally shied away from those. Big brother (CDJR) is always watching with modern ECUs. I do love Jeeps but I don't have full confidence in their engines. My 2012 JK had the head issue and my Hemi makes more ticking noise than I am comfortable with. Not handing them any excuse for engine warranty issues. I know you can reset before taking to the dealer but call me paranoid......

biochem · Nov 1, 2023

Viking Jeeper said:
Post #5 said the numbers given were not exact. If we are discussing the difference between 1.5 and 2.0 inches I think its fair to get exact numbers.

Pics aren’t the greatest but hopefully proves the point. Stock XR tires were in the barn and the lighting isn’t the best. They are also not mounted vs mounted but similar to what I remember on the measurements.

I was just trying to be helpful to the OP as I had first hand experience w the product in question.

I was surprised by the exact measurements as well when setting the new diameter on my Tazer.

Viking Jeeper · Nov 1, 2023

So you are comparing mounted vs not mounted tire size?

biochem · Nov 1, 2023

Sure. I’m not here to really prove anything. I feel like I put more effort into answering the question that I really intended already. Again, just trying to be helpful. It was stated in my post mounted vs not mounted. This forum seemed to be a friendly place but maybe I was mistaken.

The stock tire size was set at 34.something when initially loading the Tazer. Pretty in line with my measurement. The XR tires have about 11k miles on them.

The tire size is now set at 35.something in my Tazer from my new measurement on the 37 KO2s. The new tires have about 1k miles on them.

In reality, my experience says that these tires are not two inches difference in diameter. I frankly don’t have the time or care enough to try to score internet points with remounting the stock XR tires, more pics, etc.

Not sure why the presumption is that I would be untruthful on this forum or any other for that matter. At this point I’m not sure there is evidence I could present that would satisfy your curiosity.

Fast-n-Furious · Nov 1, 2023

Here’s my stock 22 XR hardtop at 30 psi 13k miles, total extra 150lb (rock sliders + rear trunk cargo gears), measure all 4 tires, 33-1/4”.

YukonCornelius · Nov 8, 2023

Un-mounted BFGoodrich specs:

Jeepherz · Nov 10, 2023

I've decided to stick to the stock tires for now. I wasn't concerned about what tire is exactly what measurement. I know that they vary. The question was what would a 37" tire- under a normal generalization of size- do to performance based on hard numbers. I figured someone had clocked it, looked at numbers etc. Not the case so she's staying stock at least until it really needs tires.