Everyday Life Calculators

Tire Size Calculator

Compare the overall diameter of two tire sizes and see how much a size change affects your speedometer.

Original diameter —

New diameter —

Diameter / speedometer difference —

About the Tire Size Calculator

Tire sizes are written as width/aspect ratio R rim size (for example 225/45R17). Changing any of these numbers changes the tire's overall diameter, which affects speedometer accuracy and effective gearing.

Formula: Diameter (in) = Rim Size + 2 × (Width × Aspect Ratio ÷ 100) ÷ 25.4

A positive percentage difference means the new tire is taller, which makes your speedometer read slower than your actual speed; a negative difference means the opposite.