I went to the dealership to see if I could have them reprogram my 98 XJ ECU to reflect bigger tires (32s) and re-geared axles (4.10).

Funny enough, the service advisor was a nice guy who drives a highly modified JK and was totally into it.

According to him, Chrysler can only program parameters that were an originally offerred option on the vehicle. So for axles, the largest offered ratio ever was 3.73, which means it's the top value they can write to the ECU. Same goes with wheels and tire sizes, etc.

Which leaves XJ owners with some very limited choices, such as Superchips, etc. I do have a Superchip for my Chevy and it works great. I modified my tire size, the shift points, fuel grade, rev limiter, etc.

Can all these be modified for an XJ as well? Does anybody know which Superchip model works for XJs. The one listyed for the JK/Dodge, did not have XJ listed in there...

Thanks.

 

There are charts online where you find your gearing and tire size, then you just buy the correct tooth speedo gear. They are about $30.

 

There is a speedo gear in your tcase that can be replaced with a different tooth count to correct the speedo

 

I thought on a 98, it is electronic. In any case I care less about the speedo and more about proper shifting and such.

My Chevy, once adjusted with the Superchip, drive much better and burns less fuel...

 

You correcting the speedo gear will fix your shifting, it let's the tranny know when to shift.

 

^^^This



pcm looks at the speedo input (as well as a bunch of other sensors), then sends info to the tcm for shifting.