It adds mileage to your car if you change something in the tuning bit before exiting, such as Brake bias, suspension etc...only need to change one variable. A couple of weeks ago I did over 1200 miles in Arcade using one car and it is showing the mileage. It doesn't count it when you look at most used mileage vehicle (mine is currently listed at 800 miles and another car), though it does count towards your total mileage overall