The tyre size as quoted by retailers and stamped on the sidewall will provide you with this information. As an example, a 205/65R15 tyre has a width of 205mm, and the 65 is the height of the sidewall as a percentage of the width, so in this case it would be 65% of 205mm = 133.25mm. On a 15-inch wheel (381mm) this combination would be 381mm + 133.25mm + 133.25mm = 647.5mm.

However, it is crucial to ensure that the spare wheel is large enough to fit over the brakes of your vehicle. We would suggest looking at retailers such as thewheelshop.co.uk who provide a range of spare wheels designed to fit a range of vehicles.