Tire Size Converter

Enter a P-metric size — section width in millimetres, aspect ratio (sidewall as a percentage of width), and wheel diameter in inches.

Enter an imperial size — overall diameter in inches, section width in inches, and wheel diameter in inches.

Compare your original tire against a new one to see the change in overall diameter and how far off your speedometer will read after fitting the new size.

Reading a metric tire size

A P-metric size such as 225/45R17 encodes three numbers:

• 225 — the section width in millimetres (tread-to-tread).
• 45 — the aspect ratio: the sidewall height as a percentage of the width. A lower number is a shorter, sportier sidewall.
• R — radial construction; 17 — the wheel diameter in inches.

The formulas

Everything else is derived from those three numbers:

• Sidewall height = width × aspect ÷ 100  (e.g. 225 × 45 ÷ 100 = 101.3 mm)
• Overall diameter = wheel diameter + 2 × sidewall  (convert 25.4 mm per inch)
• Circumference = π × overall diameter
• Revolutions per mile = 63,360 ÷ (π × diameter in inches)

An imperial / flotation size like 33x12.50R15 states the overall diameter (33 in) and section width (12.50 in) directly, so converting to metric is just a matter of turning inches into millimetres and back-solving the aspect ratio.

Speedometer difference when upsizing

Your speedometer counts wheel revolutions and assumes the factory tire diameter. Fit a taller tire and each revolution covers more ground, so you are actually going faster than the dial shows; a shorter tire does the opposite. The error is simply the diameter ratio:

actual speed = indicated speed × (new diameter ÷ old diameter)

Keeping the overall diameter within about ±3 % of stock is the usual rule of thumb to avoid speedometer error, ABS/traction issues, and rubbing.