Ok so I'm about to lift my XJ and go to 31s. I have 3.55 stock gears. Do I need to regear to get back to the stock ratio?

 

To get back to stock yea you will. From what I see most people are happy with 4.11's and 31's. I personally haven't regeared yet, but that's what I plan to run.

 

T0 run the same speed at the same rpm yes. Is your jeep going to break or will you notice a detrimental drop in performance probably not. Ive been on 31's with 3.55 for the last two years and it not bad at all. I will be getting 33's soon and I here thats still very doable on 3.55's. From what Ive seen is most jeeps dont really start begging for a regear till 35's. At that point its time to bust out the 4.88s

 

If you're going to stay 31's, then you can do 4.11's when you get around to it. 31's aren't going to hurt you too much with stock gears.

 

Will it affect my mpg much? Im getting around 16-17 mpg in town on 235/75/15s completely stock. I know when I lift it and put bigger tires on it will affect my mpg but I'm wondering how badly it will be affected.

 

You'll be fine - just be happy you don't have to deal with 3.07s. And here, look at this chart:

http://www.jeep4x4center.com/jeep-ti...?utm_source=cj

 

With 3.55 and 31's, it'll be fine unless you're going to tow a trailer. That, you will notice, but it's still OK. I tow a trailer and the only time it really bugs me is on long drives while on hills. Mostly just the drop in gas mileage.

 

Yes you'll loose a 2-3 mpg with the lift and tires.

 

Re gearing is going to run around a grand. That's a lot of gas. I'm on 33s and no plans of re gearing any time soon. My 4 banger TJ has been on 31s and stock gears for around 8 years.

 

Well "from what you have seen" I can tell you one thing mine struggled a bit on 31x10:50's with 3.55. But when I went to a 33x12:50's it was a different ballgame.

On the highway with a slight wind or even the least incline you might not be aware of you will know with the 3.55's. Your transmission will be shifting in and out of overdrive all the time. I installed an external trans temp gauge when I first got it and I will tell you it does cause it to overheat some doing that back and forth deal.

When you do a lift your XJ and keep adding options like larger tires/roof racks/bumpers/etc. it does demand a gear change. I'm trying to decide what I want 4.56 or 4.88 for mine I have the lockers and will install the gears at the same time. You can see why my jeep needs more gear I added weight and drag to it causing more problems. You can see the things that give me the problems with the 3.55 below.

So IMHO I would sat YES to the gear change look at the gear chart shown on this web site and you can see what you want.

http://

 

I'm running stock height, 3.55's and 31x10.50 mud terrains, and noticed a slight drop in fuel mileage although I can't give you an exact number because I haven't updated my speedo gear. But, I haven't really noticed much power difference compared to the 235x75x15's I had on before. My 4.0 hauls *** with 'em. My tranny doesn't hunt for gears on the highway, and all works perfectly.

 

^ This. When I put 31's on my '00, it didn't give two ****s. Even my 2.8L powered pig didn't care when I went to 31's. IMO, 33's are when you should really start looking at regearing.

 

For a split sec I thought you were talking to yourself..... Lol then realize you guys have the same name, different order lol

 

I have noticed though at any hill, I've found it to be easier to just shift in to 3rd then leaving it in drive when I'm on the highway.
City wise, I just keep it in 3rd.

 

I'm currently at 3" lift and 31's with 3.55 gears. Long hills and acceleration leaves something to be desired, if I really get on it I just burn gas and make noise. With my roof lights, and weighed down with a bumper and plenty of stuff inside, I get 13mpg. On highway I often take off my roof lights and get about 17mpg highway. I plan on running 33's and 4.56 gears sooner or later but this setup has lasted me over a year now.