Tires treads are measured at 32nds of an inch with a brand new tire being at 10/32nds, which is about 1/3 of an inch. When your tires get to 2/32nds, they are legally bald and needs immediate replacement. You can even get a ticket if you don't get them replaced. Not to mention that it is extremely dangerous to drive on bald tires.If you are unsure if your tires need replacement or not you can always use a tire gauge to check the treads. Now, most people don't have a tire gauge just laying around, but you can also use the penny trick. You insert a penny into a groove in the tire with Lincoln's head facing downward. If you can see the top of his head, you need new tires. If your tread is to his forehead, then your tires are worn and need to be replaced soon.Remember to check your tires often for your safety. If you are concerned about your tires, it is always a good idea to talk with a qualified mechanic for help.