Will MIL display when new tires are smaller?

[66chevelle]

I would like to upgrade to 18 in wheels on a 07 GMC Canyon (Colorado). Dealer specs tires that are 2 sizes too small. They say GM tested this size and it is the only one covered by warranty. They also say that the BIC would have to be reprogammed with a different tire size. Tire Rack and a tire dealer friend says stay with the correct 18 in size which the circumference matches the 15 in tire I have now. Will the MIL set when the speedometer is off by 2.1MPH? I could live with the speedometer not being exact but I don't want to have the BCI reset every spring and fall when I switch wheels and tires back and forth. I am using the 15s for snow. I hope a GM tech can answer this. Thanks.

[UncleBob]

The vehicle isn't aware of actual speed. All speed calculations are based off of tire rotation, and tire circumference won't effect that

What might happen though, (if you had a drastically different tire size) is the shift points could get confused/act odd, because the loads at speed are different that it was programmed for. You're talking such a small difference though, I doubt you'll notice any difference.

The speedo and trip/odometer will be off the same percent of course, unless you reprogram the computer for the new tire size

[curtis73]

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.