2009 389 peterbilt

My 09 389 peterbilt is clicking the miles off way to fast. It’s saying like 5 miles to every real mile. Has anybody ever had this problem and what was causing it?
I replaced the isx cummins with a 04 bxs cat and ever since I did that the mileage has been doing this. Is there’s some parameter in the ecm that needs changed or maybe the paccar computer? Thanks!