Tire circumference and revolutions per kilometre from a size marking
Turn nominal diameter into circumference and estimated revolutions, keeping millimetres, metres and kilometres consistent.
Method
A size such as 205/55 R16 gives width in millimetres, sidewall height as a percentage of width, and wheel diameter in inches. The nominal outer diameter is wheel diameter × 25.4 plus two sidewall heights. Circumference is π × diameter. Revolutions per kilometre are 1,000 divided by circumference in metres. Radius is half the diameter, so do not put radius into the diameter formula a second time.
Worked example
For 205/55 R16, one sidewall is 205 × 0.55 = 112.75 mm. The wheel is 16 × 25.4 = 406.4 mm. Outer diameter is 406.4 + 2 × 112.75 = 631.9 mm, or 0.6319 m. Circumference is approximately 1.9852 m, giving about 503.7 revolutions per kilometre. If a replacement tire has a 3% larger rolling circumference, each revolution covers 1.03 times the distance. An unchanged calibration that indicates 100 km/h would correspond to about 103 km/h under that simple model.
Checks and limits
Use Tire size comparison to compare the original and proposed sizes; keep a consistent distance unit when checking the ratio. The geometric circumference is an estimate, not the manufacturer’s measured rolling circumference. Load, pressure, tread wear, construction and speed can change the effective rolling distance. A size comparison does not establish wheel clearance, load rating, speed rating, or compatibility with vehicle systems. Check the vehicle and tire manufacturer’s specifications before fitting a different size.