Tire Size Calculator

Reading a Tire Size and How the Math Works

A tire size like 225/55R17 packs three numbers into one code: the first (225) is the tread width in millimeters, the second (55) is the aspect ratio — sidewall height as a percentage of that width — and the third (17) is the wheel/rim diameter in inches. Sidewall height in millimeters is simply width × (aspect ratio ÷ 100). Converting that to inches and doubling it (since a tire has a sidewall above and below the rim) and adding the rim diameter gives the overall tire diameter: diameter = rim + 2 × (width × aspect / 100) / 25.4. Circumference follows from diameter using the standard π × diameter, and revolutions per mile is 63,360 (inches per mile) divided by that circumference.

Why Speedometers Read Wrong After a Tire Size Change

Your speedometer doesn't measure road speed directly — it counts wheel revolutions and assumes the original tire diameter to convert that into mph. Fit a tire with a larger overall diameter and each revolution covers more ground than the speedometer assumes, so your actual speed is higher than the indicated speed (and a smaller diameter has the opposite effect). The "Actual Speed" figure above scales your indicated speed by the ratio of new-to-original diameter to approximate this error; it's a good rule-of-thumb estimate, not a substitute for recalibrating the speedometer or ECU on vehicles that support it.

Keep the Diameter Change Small

Most tire shops and manufacturers recommend keeping the overall diameter change within about ±3% of the original when sizing up or down (a common rule of thumb, not a hard engineering limit) — bigger swings can throw off ABS/traction-control calibration, speedometer accuracy, and ground clearance/rubbing tolerances. If you're planning a broader vehicle size change, the conversion calculator can help with any other unit conversions involved, and the speed calculator is useful for working through the corrected speed and travel-time effects further.