Tires should be replaced on your vehicle when you notice that your current set is starting to break down or wear down. For example, you can use the tread test to determine if your tread is getting too low. You place a penny upside-down inside the tread. If you can see any of Lincoln’s head, then it’s getting close to needing new tires. You’ll also want to watch out for cracks in the rubber or uneven tread wear.