Tire Size Calculator

The three numbers encode everything you need. 225 is the section width in millimeters, measured at the widest point across the tread. 45 is the aspect ratio: the sidewall height is 45% of the width. R stands for radial construction, and 17 is the wheel rim diameter in inches.

Sidewall height is the distance from the bead (where the tire seats on the rim) to the outer tread edge. The formula is sidewall = width x aspect / 100. For 225/45R17 that is 225 x 45 / 100 = 101.25 mm. The calculator uses this to find overall diameter: rim diameter converted to mm plus two sidewalls.

The aspect ratio sets sidewall stiffness. A low ratio like 40 means a short, rigid sidewall that sharpens steering response but transmits more road shock into the cabin. A high ratio like 65 gives a taller sidewall that absorbs bumps better and allows more flex in corners, which improves ride comfort at the cost of some handling precision.

Your speedometer counts wheel revolutions per unit of time, not actual ground speed. A larger overall diameter covers more ground per revolution, so when the speedo reads 60 mph your true speed is higher. The calculator computes your actual speed at an indicated 60 mph, and shows the percentage error, by comparing the new tire diameter to the original.

Keeping the new overall diameter within 3% of the original is the widely accepted limit. Larger deviations push the speedometer and odometer readings noticeably off, and can affect ABS and traction control calibration that rely on wheel-speed accuracy. Some states also use odometer readings during emissions or safety inspections, so significant drift can cause practical problems.