Tyre Size

Welcome to Preston Tyre Bay and Autocare. 

At the latest when buying tyres, you have to deal with the tyre size. Determining the correct tyre size of a car tyre is a challenge for many vehicle owners. 

This is due to the combination of letters, numbers and the mixed units of millimetres, inches and percent, which should be noted. We at the Preston Tyre Bay want to show you on this page what is important for you as a motorist and explain the meaning of the values. You can read your current tyre size directly on the sidewalls of the tyres or in the vehicle registration certificate. 

For the tyre size, six values are relevant. So when the tyre dealer speaks of tyre dimensions, he means exactly this information. 

1. Tyre width 

The name already tells: this is about the width of the tyre. It is measured from side to side in millimetres.  

Besides the sporty look, wider tyres offer better grip on the road. Narrower tyres have a lower fuel consumption. 

2. Aspect ratio 

The tyre cross section is the ratio between sidewall height and the width of the tread. 40, for example, means that the sidewall height is 40% of the tread width. Experts speak in this example of a "Series 40 tyre" or simply a "40s tyre". 

The lower the number, the sportier the tyre looks. 

However, low aspect ratio tyre sizes also have one drawback: they do not dampen bumps so well, so they offer slightly less riding comfort. 

3. Tyre design 

The type of tyre says something about the internal structure - i.e. the construction - of the tyre. This is primarily about the direction of the carcass's steel or nylon cords responsible for the stability/flexibility of the tyre. The letter R means Radial construction. The letter B stands for diagonal structure. 

In standard cars, almost only radial tyres are used. 

However, diagonal tyres (and their undesirable properties in cars) are, e.g. used in classic cars, some off-road vehicles and construction vehicles. 

4. Rim diameter 

The diameter of the rim expressed in inches. Regardless of the optical preferences with smaller rims and a correspondingly small tyre size, you can achieve better acceleration values, with larger tyres higher end speeds. 

Attention: For some wheels (= wheel + tyres) the rim diameter is indicated in millimetres instead of inches. Exceptions include PAX tyres, Dunlop TD tyres and Michelin's TRX and TDX tyres. 

5. Load index 

This value states with how many kilograms (max.) a tyre may be loaded at a particular tyre pressure. 

6. Speed ​​Index (SI) 

The speed index says how fast a tyre can be driven (with maximum speed). "W" for example means 270 km/h. 

We hope, you found this information useful. If you need help or have any more questions, please get in touch with us. 

Our tyre experts will be happy to help. 

Your Preston Tyre Bay team