Tire Size Comparison Calculator
Compare two tire sizes to see the change in overall diameter, sidewall, circumference, and your speedometer error. Know before you plus-size or change tires. Free.
Reading a tire size
A size like 225/45R17 means 225 mm tread width, a sidewall that's 45% of that width (the "aspect ratio"), on a 17-inch wheel. The overall diameter is what actually matters for gearing and your speedometer — and two very different-looking sizes can have nearly the same diameter, which is the whole point of a "plus size" swap (bigger wheel, shorter sidewall, same rolling diameter).
The ±3% rule
Keep the new tire's overall diameter within about ±3% of the original. Beyond that, your speedometer and odometer read wrong, ABS and traction control get bad wheel-speed data, gearing/fuel economy shift, and you risk rubbing at full lock or compression. This calculator flags anything over that threshold. After any tire change, it's also worth resetting or relearning the TPMS sensors.