Aspect Ratio is a tire’s section height expressed as a percentage of its section width, and it is the second number in a metric tire size. A 315/30-18 tire has a sidewall height equal to thirty percent of its 315 mm width, roughly 94.5 mm, so a lower number means a shorter sidewall for the same width. Lower aspect ratios stiffen the sidewall, which reduces contact patch distortion under lateral load and produces quicker, more direct steering response, at the cost of ride harshness and less protection for the wheel against impacts. Aspect ratio also feeds directly into overall diameter, so changing it while keeping the same wheel and section width alters rolling radius, effective gearing and speedometer calibration.