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Tyre Size Speedo Calculator

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.

Enter the old and new tyre sizes below for comparison:
Size 1Size 2Width
Aspect Ratio
Rim Diameter

How To Use The Speedometer Calculator

Here's how to use the calculator properly:

  1. Enter your existing tyre size (width, aspect ratio and rim diameter)
  2. Enter the new tyre size (width, aspect ratio and rim diameter)
  3. Check out the result in the section below. It will show the percent difference between the speedometer reading and your new, actual speed.
  4. Reference the table to see what the speedometer will read at various actual vehicle speeds.

Why Does The Speedometer Need Calibrating?

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.

How Much Does The Speedometer Change?

Here's how much the speedo needs correcting when changing between popular sets of similar tyre sizes:

  • 265/65R17 to 265/70R17: +3.4%
  • 265/70R16 to 265/75R16: +3.4%
  • 285/75R16 to 265/75R16: -3.6%
  • 285/75R16 to 305/70R16: -0.1%
Louis from Tyre Geek


I'm Louis, an engineer passionate about helping Australians choose better tyres for their vehicles!