Tyre Size Calculator

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Sidewall, overall diameter, circumference and revolutions per km from a marking like 185/65 R15 — and two sizes compared with the speedometer difference

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Enter the values

What to work out

Width (185 in 185/65 R15)

Aspect ratio (65)

Sidewall height as a percentage of the width

Rim diameter (15)

inch

Result

Overall diameter

621.5mm

Revolutions per km

512.2

Overall diameter: 24.469 inch
Sidewall height: 120.25 mm
Circumference: 1,952.5 mm

Working

185/65 R15:
h = 185 × 65 ÷ 100 = 120.25 mm
D = 15 × 25.4 + 2 × 120.25 = 621.5 mm
C = π × D = 1952.5 mm; rev/km = 1 000 000 ÷ C = 512.2

Formulas

h (sidewall) = width × aspect ÷ 100
D = rim × 25.4 + 2h
C = π D
revolutions per km = 1 000 000 ÷ C (mm)
v_actual = v_shown × D_new ÷ D_old

h: sidewall height (mm)
D: overall tyre diameter (mm)
C: circumference (mm)

Assumptions

Nominal, unloaded dimensions from the metric marking (width/aspect R rim). Under the vehicle's weight the rolling radius is a little smaller, and a worn tread is smaller too.
The speedometer is taken to be correct on the old tyre.

Sources: Kirpal Singh, Automobile Engineering (Vols. 1 & 2) · ISO metric tyre size marking (ISO 4000-1, ETRTO)

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