Worst case scenario, you may be just a few (1-3mph) off of your actual speed. You'll only see problems with speed calibration when you change rear end gears or go with HUGE rims. If your car had 17" rims from the factory, the extra inch isn't going to change anything really. If you originally had 15" or 16" rims than you still won't be far off....not far enough to worry about it anyway. If you went from 15" rims to big 22's or something like that you can have problems. You can recalibrate you speed in most tuners if you have one. They also make a plug in "speed cal" that corrects even the slightest issue. I'd save your money though.