Tyre sizes explained

Most manufacturers follow an industry-standard method of indicating tyre size with a series of letters and numbers, so you’ll be glad to hear that no matter which manufacturer made your tyre, its size can be found embossed or printed on the tyre sidewall. These numbers indicate the size, type, and performance of the tyre.

Now you’ve located the series of letters and numbers on the side of your tyre, let’s read it together using the image above as our example.
The tyre size is: 225/45 ZR17 94Y.

The first number represents the width of the tyre in millimetres from one sidewall to the other, meaning this tyre’s radial width is 255mm.

After the slash is the height/profile ratio as a percentage of the width of the tyre. In this case it would be 45. This number is calculated by dividing the tyre’s section height by the tyre’s section width, so if the tyre has an aspect ratio of 45, the tyre’s height is 45% of its width.

Next are the letters ‘ZR’ which indicates the type of tyre dependent on how it was constructed. The 'Z' before the 'R' just means that this tyre has been adapted for speeds above 149mph. On this occasion, the ‘R’ specifically refers to the tyre being a radial type, which is generally the most common type of tyre you’ll see on the roads these days. However, alternative letters can be used to indicate other types of tyres - B (Bias-ply) and D (Diagonal construction).

Following the letter ‘R’ is the number ‘17’, which tells us the size of the wheel rim diameter stated in inches. Therefore for our example, the tyre will fit over a rim that is 17 inches.

The ‘94’ indicates the load index, which is the maximum load that the tyre can support when inflated properly.

Ending the sequence is the letter ‘Y’ which relates to the tyre speed rating. In our example, the speed rating indicates a maximum speed of 186 MPH (300 kilometres per hour).