The trucks computers are a bit more forgiving, but when I went from a 27" circumference to a 25.8" circumference I had to do some pretty radical changes to shift points. It read the smaller tires as less torque input and shifted WAY early. It also is still very slow to downshift. I really have to get into the pedal to get it to downshift. I'm now using a 26" circumference and I'm closer to normal, but I'm not going to spend too much time reprogramming because I have a 6-speed ready to put in.
