Tyre size calculator

About this calculator

Enter your current and new tyre sizes as printed on the sidewall and the speed on your speedometer. The calculator gives the sidewall height, overall diameter, revolutions per mile and the real speed.

Keep the overall diameter within about 3% of the original. Check your vehicle handbook before changing size.

Measurements and terms explained

Width

The tyre's width in millimetres from sidewall to sidewall.

Aspect ratio

Sidewall height as a percentage of the width.

Rim size

The wheel diameter in inches.

Overall diameter

Rim size plus twice the sidewall height.

Revolutions per mile

How many times the tyre turns to cover a mile.

Common questions

What do the numbers on a tyre mean?

In 225/45R17, 225 is the width in mm, 45 is the sidewall height as a percentage of width, R means radial and 17 is the wheel size in inches.

Will a bigger tyre change my speedometer?

Yes. A larger diameter means the car travels further per wheel turn, so the speedometer reads lower than your real speed.