The size of car and light truck tires is usually expressed in a short series of numbers and letters that read like this (as seen in the above illustration): 245/40-R18. The first number indicates the width of the tire at its widest point in millimeters. The number after the slash is what's called the aspect ratio, which indicates how tall the sidewall is as a percentage of the width. The higher the number, the taller the sidewall. The squat tires fitted to sports cars and muscle cars, called low-profile tires, have aspect ratios of 30–40.