Can I run 31" tires on the stock gears of a TJ "X"?  

They will fit but they will rub when twisted up or turned to full lock. Your gears may be a little tall.

 Buy them put them on, if you don't like what you have get a coil spacer lift. Then regear the axels.

No. Well, you can turn them, but no more acceleration or using OD like you used to, and hills are a bear. Mileage will drop as you try to drive it normally. Your X has 3.07s stock, unless you upgraded to ABS. I currently run 31s, and put in 3.55s. Work like a charm, pretty close - a bit over - to stock, and the extra gearing is good for the extra rolling resistance. But naturally it depends what you do with your jeep. Mine's mostly dd, with 4WD when it's needed, rather than for fun (except occasionally). So my setup (I also have truetracs front & rear) works well for me. But if yours is a weekend 4 wheeler, with some dd use, then go to 3.73s for the 31s.

This only applies to the 5-speed, by the way, don't know about the 6-speeds.

Did you do the re-gear job yourself?  

No, I believe that's there's a little more knowledge than I have to setting the gears. There a subtlety to the backlash that can cause problems if it's wrong, torquing up everything seems to have a little bit of a subtle touch, borne of experience. Once upon a time, I might have read up and tried, but no longer. I've drug out and put back quite a few diffs, removed broken axle ends, etc, but as far as replacing the gears in the carrier - different thing. However, looking around and see who is doing it must say something .... On the other hand, all these people have learned on someone else's iron, and dollar. You might want to sidle up to a friendly mechanic and look at what they do, help a bit, and see the tricks. Then do your own. Shoot, it's just metal. But when you see an 110V electrican connecting a few wires, what's the problem? Then read about wiring, and a whole new world opens up. It seems that there's a bunch of stuff we do that is the equivalent of connecting up an overhead fan, and then there's installing a circuit board with the right balance of load.

But I'm also a bit lazier now, and, compared to times past, have a few more dollars. Or did.

i have an 06 Wrangler X, 6 speed with 3.07 gearing. I use mine as a daily driver and have noticed really no difference in power. I can still run at 70-75 easily in 6th on the hwy. Around town I only feel a moderate loss of torque, it will still get up a go if i need it too. i think going to 3.55 woul be necesary if I towed more or did use it for off raod more.  

I have 31's on my 96zj with the 4.0 and I think it works just fine, I have no trouble passing on the highway. i wouldn't waste money on re gearing untill you get to 32/ 33" tires....I'd buy a BB lift if I was you and the 31's... if after that YOU yourself feel you need more power then go for it.  

OK,

I'm new to the forum and admittedly not a mechanic at all.  But I LOVE my 2003 TJ "X".  I am just starting to do some minor modifications to it other than cosmetic.  This week I'm adding a 2.25 in. lift kit.  (OK, I'm having my local mechanic do it)  But what I'm not getting is if I want to go to bigger tires such as 31's or even 33's what else has to happen? 

Sorry for my ignorance, I'm learning here.  Thanks.

if you just want 31" go for it now it will make your truck look way cool and help off road.  any biger and youll need to regear your axels (expensive).  with 31"s you may notice a small hit in acceleration and fuel economy  but if you wanted to sip gass youd drive a prius. also going biger than 31 would probably require more lift to prevent rubbing at full lock and stuff (full turn of the wheel and compression of the suspension). goodluck and safe and legal wheeling....  

Thanks a lot.  That helps.   

I've got a question. 4" Rancho Suspension lift on 2000 wrangler 2.4 liter, with 33x12.5"s,3 spead Auto? I have someone who wants to swap outright for my stock Suspension and Tires. Will this be too much on the engine, I believe it has 3.73 gears. I would appreciate your advice or anyones for that matter. It's a heck of a deal but just want to make sure.  I dont want to mess anything up on it. The other jeep is a 4.0.

Richard