Determining what size tires are on your car is a quick and easy thing. Go to your car, and look on the sides of your tires. You will see a number of things. For instance, the name a of the manufacturer of the tire (i.e. Goodyear). You will also see some combination of numbers and letters that look something like this: P205 80/R17. The 205 has to do with the circumference, the 80 is how wide, and the 17 is the size of the rim.