Use the comparison tool to compare factory and replacement tyre sizes. The calculator automatically parses common tyre formats, calculates overall diameter differences, circumference differences, speedometer error and ride-height changes.
Keeping tyre diameter close to the original specification helps maintain vehicle handling, gearing, ABS performance and speedometer accuracy.
A tyre marked 225/45R18 has a width of 225 mm, an aspect ratio of 45%, and fits an 18-inch wheel.
This calculator can determine sidewall height, aspect ratio, overall tyre diameter and rolling circumference from the values you enter.
Sidewall height is calculated by multiplying tyre width by the aspect ratio percentage.
Overall tyre diameter equals wheel diameter plus two sidewalls.
Circumference is important because it affects gearing, speedometer accuracy and distance travelled per wheel revolution.
This calculator includes presets for Ferrari California, Ferrari 458, Ferrari 328, Porsche 911, Tesla Model Y, Vauxhall Astra, 2003 Vauxhall Corsa, quarry haul trucks, Ferrari Formula 1 cars and the famous Bigfoot monster truck.
Changing tyre diameter can alter ride height, acceleration, speedometer readings and fuel economy.
Many enthusiasts use tyre calculators when upgrading wheels to ensure the overall rolling diameter stays close to the factory specification.
Large changes in diameter can affect ABS systems, traction control systems and gearbox calibration.
For best results, aim to keep the overall tyre diameter within a few percent of the original manufacturer specification.
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