Motorcycle Tyre Size Calculator
Calculates overall diameter and circumference from a tyre size and the speedometer error between two tyres.
Free · No sign-upA motorcycle tyre size has three parts: tread width (mm), aspect ratio (%) and rim diameter (inches). For example, 120/70-17 means 120 mm wide, a sidewall 70% as tall as the width, and a 17-inch rim. 'R' marks radial and '-' bias-ply construction; neither changes the size.
Fitting a different size changes the wheel's circumference. Because the speedometer counts wheel revolutions, the indicated speed and distance change too. This calculator compares two tyres to show that error.
Current tyre
Printed on the sidewall, e.g. 120/70-17
- Sidewall height
- 84 mm
- Overall diameter
- 599.8 mm
- Circumference
- 1,884 mm
- Revolutions per km
- 531
New tyre
Printed on the sidewall, e.g. 120/70-17
- Sidewall height
- 96 mm
- Overall diameter
- 623.8 mm
- Circumference
- 1,960 mm
- Revolutions per km
- 510
Speedometer error
Diameter difference
+4%
Difference above 3%: check the sizes approved in your manual.
Real speed when the speedo reads 100
104 km/h
Speedometer calibrated for the current tyre
Speedo reading at a real 100
96.2 km/h
What the speedometer will show with the new tyre
How it is calculated
Sidewall height = width × aspect ratio ÷ 100. Overall diameter = rim diameter × 25.4 + 2 × sidewall height. Circumference = π × overall diameter.
If the new tyre is larger, one wheel revolution covers more ground: when the speedometer reads 100 km/h, the real speed is 100 × (new diameter ÷ old diameter).
In inch sizes such as 3.00-10 or 2.75-17 the width is given in inches, and on these traditional tyres the sidewall is taken as roughly equal to the width (~100%). The result for inch sizes is therefore approximate.
Formula
- Sidewall (mm) = width × aspect ratio ÷ 100
- Diameter (mm) = rim (in) × 25.4 + 2 × sidewall
- Circumference (mm) = π × diameter
- Real speed = indicated speed × (new diameter ÷ old diameter)
Worked example
- 1120/70-17: sidewall 120 × 0.70 = 84 mm; diameter 17 × 25.4 + 2 × 84 = 599.8 mm; circumference ≈ 1,884 mm
- 2120/80-17: sidewall 120 × 0.80 = 96 mm; diameter 623.8 mm; circumference ≈ 1,960 mm
- 3Difference: 623.8 ÷ 599.8 = 1.040 → diameter 4.0% larger
- 4When the speedo reads 100 km/h the real speed is ≈ 104 km/h; at a real 100 km/h the speedo reads ≈ 96 km/h
Good to know
- Sizes outside the manufacturer's approved list can affect handling, how ABS reads wheel speed, and clearance to the mudguard, swingarm or chain. Stick to the sizes in your owner's manual.
- Only the tyre on the wheel the speedometer reads from (the front wheel on most models) affects the speedometer.
- The real rolling circumference under load is a few percent smaller than the calculated value because the tyre deforms; the percentage difference between two tyres still holds.
Frequently asked questions
What is the diameter of a 120/70-17 tyre?
About 60 cm. The sidewall is 120 × 0.70 = 84 mm, so the overall diameter is 17 × 25.4 + 2 × 84 = 599.8 mm.
How does a higher-profile tyre affect the speedometer?
Fitting a 120/80-17 instead of a 120/70-17 makes the diameter 4% larger, and the speedometer reads about 4% below the real speed: at a real 100 km/h it shows about 96 km/h.
What do R, ZR and '-' on a tyre mean?
R means radial construction and '-' means bias-ply. ZR marks radial tyres rated for speeds above 240 km/h. Load capacity and speed rating are shown separately by a load/speed index such as '58W'.
Where can I find my tyre size?
It is printed on the sidewall (e.g. 130/70-12 56P). The approved sizes for your motorcycle are in the owner's manual and, on most models, on a tyre label on the swingarm, chain guard or under the seat.
Results are for information only and are based on theoretical calculations. For parts and maintenance, follow your motorcycle's owner's manual and the manufacturer's specifications. Last reviewed: October 8, 2026
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