for sake of round numbers...lets say you have 16 inch rims....the tires you get might be 205/40/16. the first of the three numbers (205) is hoe many millimeters wide the tire is....the second number (40) is the percent of the width that is the side wall...so...take 40% of 205...and thats how many millimeters tall your side wal is...and finally the last number (16)....is rim size. congratulations you just passed tire 101.