Tire Size Calculator
Compare two tyre sizes: overall diameter, sidewall height, circumference and how much your speedometer changes.
Only fit tyre and rim sizes approved for your vehicle (see the registration document or the manufacturer). Check load index, speed rating and wheel-arch clearance with a tyre dealer.
About this tool
Enter your current tyre size and the size you are considering (for example 205/55 R16 and 225/45 R17) and see the overall diameter, sidewall height, circumference and revolutions per kilometre of both. The calculator shows the difference in millimetres and percent and what your true speed is when the speedometer shows a given speed on the new tyres. Diameter = rim × 25.4 + 2 × width × aspect ratio ÷ 100.
Frequently asked questions
205 is the tyre width in millimetres, 55 is the aspect ratio — the sidewall height as a percentage of the width (55 % of 205 mm = 112.75 mm) — R means radial construction, and 16 is the rim diameter in inches. A following code such as 91V gives the load index and speed rating.
Convert the rim size to millimetres (× 25.4) and add the sidewall twice: diameter = rim × 25.4 + 2 × width × aspect ÷ 100. For 205/55 R16 that is 16 × 25.4 + 2 × 112.75 = 406.4 + 225.5 = 631.9 mm, or about 24.88 inches.
The speedometer counts wheel revolutions, so a larger tyre covers more ground per turn. True speed = shown speed × new diameter ÷ old diameter. Going from 631.9 mm to 650 mm (+2.9 %) means 100 km/h on the speedometer is really about 102.9 km/h. Speedometers built to UN Regulation 39 may read high but never below the true speed.
No. Use sizes approved for your vehicle — they are listed in the registration document or the certificate of conformity, or approved by the vehicle or tyre manufacturer. The load index and speed rating must also be at least what the vehicle requires, and the tyre must not touch the wheel arch or suspension.
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