Free Tire Size Calculator

Compare two tire sizes and see the difference in diameter and speedometer accuracy.

Tire size notation encodes width (mm), aspect ratio (sidewall height as % of width), and wheel diameter (inches) — changing tire size affects your rolling diameter, which throws off your speedometer/odometer accuracy proportionally if not recalibrated.

Worked Example

Example: 225/50R17 has a diameter of about 25.1 inches. Compared to 235/45R17 (24.8in), that's roughly a 1.2% smaller diameter — your speedometer would read slightly higher than actual speed.

Formula / Method

Sidewall Height (mm) = Width × (Aspect Ratio/100)
Diameter (in) = Wheel Diameter + 2 × (Sidewall Height / 25.4)

Worked Example

Example: 225/50R17 has a diameter of about 25.1 inches. Compared to 235/45R17 (24.8in), that's roughly a 1.2% smaller diameter — your speedometer would read slightly higher than actual speed.

Formula / Method

Sidewall Height (mm) = Width × (Aspect Ratio/100)
Diameter (in) = Wheel Diameter + 2 × (Sidewall Height / 25.4)