# Motorcycle Tyre Size Calculator > Calculates overall diameter and circumference from a tyre size and the speedometer error between two tyres. A 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. ## 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 1. 120/70-17: sidewall 120 × 0.70 = 84 mm; diameter 17 × 25.4 + 2 × 84 = 599.8 mm; circumference ≈ 1,884 mm 2. 120/80-17: sidewall 120 × 0.80 = 96 mm; diameter 623.8 mm; circumference ≈ 1,960 mm 3. Difference: 623.8 ÷ 599.8 = 1.040 → diameter 4.0% larger 4. When 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. - **Page:** https://motorsikletparcasi.com/en/tools/tire-size-calculator --- - Source page: https://motorsikletparcasi.com/en/tools/tire-size-calculator - Last updated: 2026-10-08 - Türkçe sürüm: https://motorsikletparcasi.com/araclar/lastik-olcusu-hesaplama.md - Site overview: https://motorsikletparcasi.com/en/llms.txt - Append .md to the URL of any product, category, motorcycle, brand, blog or company page on this site to get its Markdown version.