Tire Size Calculator
Compare two tire sizes and see the difference in diameter and speedometer effect.
Changing to a different tire size than stock can throw off your speedometer accuracy and effective gearing — this compares two tire sizes and shows the real-world difference.
How tire size codes work
A size like 225/65R17 breaks down as: 225mm width, 65% aspect ratio (sidewall height as a percentage of width), and a 17-inch wheel diameter — changing any of these numbers changes the tire's overall diameter, which is what actually affects speedometer accuracy.
Frequently asked questions
How much tire size difference is considered safe?
A common rule of thumb is keeping the overall diameter difference within about 3%, though always check your vehicle manufacturer's specific tolerance and consult a tire professional for larger changes.
Why does my speedometer read differently with new tires?
Your speedometer is calibrated for your original tire diameter — a larger tire travels farther per rotation, so your actual speed is higher than what the speedometer displays, and vice versa for smaller tires.