I'm preparing to lift my 98 Xj 3 inches and put 31's on it. Whats the best way to make adjustments so as to make the ECM think everything is just the same as stock. I'm sure this has been covered but I searched to no avail. So sorry if I'm being redundent. I'd like to keep the already not great gas milaege from dropping too much and keep as much of the stock power as possible. Thanks in advance.

 

Your ecm does not care if you are lifted or not,but your tire size change can only be corrected by gearing or a speedo shop able to manipulate the speed to think nothing has happened,you will read about 6mph slower than what you are actually going with 31s and stock gears,assuming 3.55,it really is not an issue.

 

I think that answers my question, I appreciate the input.

 

I do believe I'm running 3.55's from the research I've done, what would the recomendation be as far as a gear swap in your opinion?

 

It depends on what you want to do with it,3.55s are fine for 31s and 32s,after that 4.10s. if it is light wheeling and dd,stick with what you have. Just my opinion.

 

Assuming you have an auto:

3.73's for 31" tires, 4.10's for 32", 4.56 for 33's##, and 4.88's for 35's

You'll have to swap in the correct speedo gear anytime you change the gears and/or tire size if you want the TCU to shift the tranny correctly.

Suggesting that someone stick with stock gears when they've changed tire size is foolish.

 

I put 30X9.50's on my 99 with stock gears (3.55). Guess what? My speedo is now spot on with the police radar mounted on trailers. I also have a 2" budget lift and no rubbing. I think this is the minimum lift for 31's and may still rub due to the small fender wells on a XJ.

 

I have 235-75-15 on mine with stock gears 3:73 I think (i have factory tow package)Im about 6 mph off or so,says I am going faster than I am.

 

you have 3.55s i dont think 3.73s were offered in the XJ.

i could be wrong.

4.0 auto = 3.55
4.0 5sp = 3.07
2.5 auto = 4.56 (super rare)
2.5 5sp = 4.10


3.73s were offered in a some ZJs

 

No they where, just not very common.

 

On a few XJs with towing package

 

mine had 3.55 with 31,s and I have no problems just do the speed limit and you wont have no problems I just back it down a little around schools,
Just a note, By federal law your suppose to be allowed 5 MPH for mechanical error, but that don't mean you wont find a cop that still wont mess with you for the fabled 2 over

 

i have 3.55s with 5" lift and 31's. my speedo is right on as well and my gearing pushes the 31's no problem whether on road or off in 4 lo or 4 hi. i dont think anyone should bother regearing with 31's. just my opinion

 

Thanks to everyone who has replied I appreciate your knowledge. My main concern was altering something that would affect shift points. This sight rocks! Thanks for having me and putting up with us new guys lack of knowledge on some things.