As car tires begin to bald, your car will have less grip on the road. Worn tires will have a harder time stopping and may slip in the middle of a turn--both of which are safety concerns. A good way to gauge your tire's treading is to use a penny. Turn Lincoln's head upside down and place it in the groove of the tread. If you can see his whole head, your tires need to be replaced. If you live in a snowy area, you may need to replace your tires even sooner.