I put a 4.0 motor and trans from a 91 XJ in my 94 XJ. It only has 125,000 miles. I'm still using the ecm from 94. I'm running 31x10.5x15 mickey Thompson baja claws. With that being said, why am I only getting 11 mph? Is it my tires or do I need the 91 ecm? Jeep runs excellent.

 

You can only go 11 mph?

 

But seriously, getting 11 MPGs is not uncommon (I get about that much as well), especially on an XJ that is not 100% stock. So yes, the bigger tires have something to do with it. Do you have a lift as well? That'll effect it. There are 100 things that affect MPGs. Start searching and start tuning up the rig. I highly doubt swapping the ECM would make any difference.

 

You have to be sure the speedo is correct. With the larger tires you will register fewer miles driven than you've actually travelled. And yes, your ECU/PCM is just fine and dandy.

 

Yes I have a 3" lift. I figured it was mostly tires, but I went from 17-18 mpg to 11-12 mpg. So I just thought I would ask. You gotta pay to play. Thanks.