tire size and gears

[dutchman]

I am running 34s on my JK right now, but am thinking about going up to 37s, can anyone tell me if I need to change my gears, or any other mods?  I have a Superchips programmer, so I can program the tire size in, but was sure if I need to do the gears.

[MrMindless]

What gears do you have now?

What are your axle plans? 37s are getting pretty big for stock parts...

Are you happy with performance now or do you already think it's a little lacking?

[Wingman]

Although not required, you'll be much happier if you do a gear swap.  That is a pretty good jump in size.

[smichaelR22]

6 speed or auto?  dont forget about trans cooler upgrades in an auto if pushing stock gears....

[dutchman]

It is stock gears, and I may want to upgrade axles at some point, but if I need to up grade gears, I may decide to do gears and axles first.  I will look into the transmission cooler, probably not a bad idea to ad one before any other mod

[JWOK]

First off you should find out what ratio you have, Either 3.23 or 4.10 I believe were the two options. If you have 3.23 you will have to change carriers as well as gears, if you have 4.10's like I did you will want to step up to 5.13's.  I currently run 5.13's auto on 35's with superchips trans cooler and it is not bad to drive now.  If you go up to 37's it will be ok but still a slug.  Other things to start thinking about when putting 37's on that d30 is axle housing reinforcements like a truss,sleeves and c-gussets. The d30's are known for bending the pass side tube and c's in the JK's when running large tires. Any questions let me know.

[dutchman]

Allot of good information, thanks.  How do I find out which gears I have on my JK?  Also the 37" tire idea is on the back burner for now, I am concentrating on the under cairrage before up grading tires, and it sound like I also need to look at axles and gears before tires.  Thanks again

[GBR928]

This chart may help you find out where you need to be when running 37's.

http://www.superioraxle.com/feature.html

[MrMindless]

I would not run anything beyond a 4.56 in a d30. I've seen too many 4.88 gear failures.

Rubicon axles with the d44 gears I might go 5.13, I'd have to think about that...

[Dragon]

I'm assuming since you have a Superchips that works that you have a pre-2011 JK.
Try this....it might give you the original window sticker for your Jeep (which should list the gear ratio):
1) Get your VIN number.
2) Copy/Paste (DO NOT HIT ENTER YET! You will just get a "broken link" message.) this into your web browser: http://www.jeep.com.hostc/windowSticker.do?vin= (enter VIN # here)
3) Enter the vin number you just copied EXACTLY right after the "=" sign on the link above. No spacing.
4) Hit enter
5) There is your window sticker specifically for your vehicle.

On the JKs, there were really 3 options.  3.21 for the basic X model, 3.73 for the towing package, and 4.10 for the Rubicons.  2007 JKs were able to get the 4.10s with the towing package, but Jeep stopped that after 2007.
The towing package also usually gave a D44 rear axle, and all but the Rubi had D30 fronts.

the consensus over at the JK forums is that 37s should have a 5.13 ratio if you have an auto.  you can get away with the 4.56 with a 6-speed, but 6th gear is almost useless except on flat highways.
Just remember....even underpowered, the JK can snap D30s with 37" tires.  If you can't afford an axle upgrade to D44s, you might want to look at chromoly axle shafts and some axle braces to stiffen the outer tubes.

if you DO have a 2011 or 2012...there are very few ways to change the ratio in the computer right now.   Chrysler changed the encryption between the engine CPU and body CPU, and aftermarket programmers are having a hard time breaking it.

Hope this helps.