Tire Size Calculator

Tire sizes follow a standard format: 225/45R17

• 225 = Section width in millimeters (the tire's width from sidewall to sidewall)
• 45 = Aspect ratio (sidewall height as a percentage of width). 45 means the sidewall is 45% of 225mm = 101.25mm tall.
• R = Radial construction (almost universal in modern passenger tires)
• 17 = Rim diameter in inches

You can find your tire size on a sticker inside the driver's door jamb, in the owner's manual, or molded on the sidewall of your current tires.

Speedometer calculator: Enter your OEM (factory) tire size and your new tire size. If the new tire has a larger overall diameter, your speedometer will read lower than your actual speed. If smaller, it reads higher. Most states allow up to 3% error without inspection failure, but check your local regulations.

Sidewall Height (mm) = Width × (Aspect Ratio / 100)
e.g., 225 × (45 / 100) = 101.25 mm

Overall Diameter (mm) = (Rim Diameter × 25.4) + (Sidewall × 2)
e.g., (17 × 25.4) + (101.25 × 2) = 431.8 + 202.5 = 634.3 mm

Circumference (in) = Diameter (in) × π
e.g., (634.3 / 25.4) × 3.14159 = 24.97 × 3.14159 = 78.47 in

Revolutions per Mile = 63,360 in/mile ÷ Circumference (in)
e.g., 63,360 / 78.47 = 807.4 rev/mile

Speedometer Error (%) = (New Circumference / OEM Circumference - 1) × 100

The 25.4 conversion factor (mm per inch) is exact by definition. Rim diameters in tire sizing are always in inches, which is why the conversion is needed to get a uniform diameter in millimeters.