Take the penny test. Simply insert a penny, upside down, into your tire's tread groove. (Lincoln should be facing you.) If you can see all of Lincoln's head, you need new tires. Check your tread depth every 3,000 miles. To save money, also check to see if your current tire is under warranty before making a purchase.
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All the information you need appears on the side of your tire. The basic tire size looks like this: P215/65R15 98H. Depending on what type of vehicle you drive, you may see an LT (Light Truck) instead of a P (Passenger).
Find Out How To Read Your Tire.