Tyre Size Calculator
Calculate tyre rolling circumference, compare street and racing slick dimensions, get speedometer correction and find the correct rim width for any tyre. Works with standard tyre sizes, slick mm format and slick cm format — including racing slicks like 30/68-18.
How to Use the Tyre Size Calculator
Select your original tyre size using the dropdowns — width in millimetres, profile as a percentage and rim diameter in inches. This is the standard format you find printed on any road tyre sidewall, for example 225/45R18. Then select or enter your new tyre size in the same way. The calculator instantly compares both tyres and shows you every measurement that matters.
If you are working with racing slicks, toggle the input mode to Slick (mm) or Slick (cm) depending on the format marked on your tyre. Slick mm format uses millimetres for both width and outer diameter — for example 250/650-18 means 250mm wide with a 650mm outer diameter on an 18 inch rim. Slick cm format uses centimetres — for example 30/68-18 means 30cm wide with a 68cm outer diameter. You can mix formats freely, comparing a standard road tyre against a racing slick in the same calculation.
FAQ
What is rolling circumference and why does it matter?
Rolling circumference is the distance a tyre travels in one complete revolution. It directly affects your speedometer reading, your gearing and the number of ABS pulses per kilometre. Fitting a tyre with a different rolling circumference than the original will cause your speedometer to read incorrectly and can affect traction control and ABS calibration.
How do I read a racing slick tyre size like 30/68-18?
Racing slick sizes are written as width / outer diameter / rim diameter. In the cm format used by Michelin and others, 30/68-18 means 30cm wide (300mm), 68cm outer diameter (680mm) on an 18 inch rim. In the mm format, 250/650-18 means 250mm wide with a 650mm outer diameter on an 18 inch rim. This is different from road tyres where the second number is a profile percentage, not an outer diameter.
What rim width do I need for my tyre?
The recommended rim width is calculated from the tyre width using a factor validated against OEM fitment data from major manufacturers. The ideal rim width is shown along with an acceptable range of plus or minus half an inch. Running a tyre on a rim outside this range affects the tyre profile, contact patch shape and handling characteristics.
How does changing tyre size affect my speedometer?
A larger tyre travels further per revolution, so your actual speed will be higher than your speedometer reads. A smaller tyre travels less per revolution, so your actual speed will be lower. The speed comparison section shows the exact difference at any reference speed you choose.
Can I use this calculator for track day and motorsport tyre selection?
Yes. The slick calculator modes are designed specifically for motorsport applications. Whether you are buying used racing slicks and want to know if they will clear your wheel arches, comparing different slick sizes for a specific class homologation, or working out gearing changes based on tyre diameter — this calculator handles all of those cases.