I have 35in tires on my jk and the speedometer and automatic shift points have been terribly off. On the highway, when my speedo read 65mph I was really going 72mph, the jk would also downshift all the time or stay in 3rd or 4th gear on level ground. the mpg's were horrendous and i always had to manually control the gears with the auto stick feature in order to improve efficiency. i did some research and found out about the procal module. i just reprogrammed the computer to account for my tire sizes and what a huge difference it makes. the speedo is right and the shifting is smooth and shifts at the right points. the dealer would have fixed this for me, but i understand that everything will go to a factory default if the battery ever goes dead. so i coughed up the $150 and couldnt be happier.
anyone who has larger tires should def look into getting one of these, im sure the fuel savings will pay for itself.
the only downside? now my odometer is accurate too, so ill have to stick to the 30,000 mile warranty instead of the secret 33,000. i did manage to rack up 270 undocumented miles though. hehe