Changing to a different tyre size and wondering how much it'll affect your speedometer?
Use this calculator to work out how much adjustment is necessary.
Here's how to use the calculator properly:
When you change tyre sizes, you change the circumference of the tyres (this is the measurement of the distance around the tyres).
When the circumference changes, the tyre travels a different distance each time it completes one revolution.
As your speedometer doesn't actually measure the vehicle's speed, but calculates it from checking how many times the tyres rotate, this calculation will now no longer be correct.
To make the speedometer reflect the true speed of the vehicle, you need to calibrate it using the calculator above.
Here's how much the speedo needs correcting when changing between popular sets of similar tyre sizes:
I'm Louis, an engineer passionate about helping Australians choose better tyres for their vehicles!