How to find out the size and type of tyres your car needs

Each type of vehicle requires a particular size and type of tyre. Tyres can be of various widths, heights and diameters, as well as varying in their load capacity and speed ability. To find the right size of tyre for a car, consult the owner's manual or look at the sidewall of a tyre to get the correct size.

Reading the sidewall of a tyre

A tyre has a lot of information printed on it. For example, you might see the following string -
255/55 R18 105 V. This means that 255 is the tyre width in millimetres and 55 is the tyre aspect ratio (height to section width). The R is the construction type, which in this case is Radial, and 18 is the internal diameter in inches. The load index is 105 giving the tyre a maximum load of 925 kg. The V indicates that the tyre has a maximum speed of 149 miles per hour. Other markings will indicate the manufacturer, how the tyres perform in winter conditions, and whether the tyre is tubeless or not.

Tyre size

When tyres have not been fitted as a complete set of four, the tyres should not be mixed across an axle. It is generally recommended that for 4x4s all the tyres are exactly the same unless the manufacturer specifies otherwise.

Changing the size of a car's tyres changes the amount of rubber that is in contact with the ground. This will result in better steering and better cornering.

Low profile tyres with short sidewalls can look good, but generally it also results in a harder ride and can adversely affect braking, fuel consumption and engine performance. If you do switch to a lower profile tyre ensure that the rim size compensates for the shorter sidewall height and stick to within +/- 2 per cent of the standard tyre. When replacing tyres, always ensure that the speed rating is the same or higher, even if the size is different.

Car usage and tyre type

A car's usage can require different tyres. If a car is largely driven in the city it is recommended that tyres should have low rolling resistance to reduce fuel consumption and good longevity due to all the stopping and starting. It is also good to choose tyres which have the optimum braking distance for dry and wet weather. For motorway driving, tyres with a good braking distance at maximum speed, comfort and good road-holding, are essential. For a sports car, look for tyres with excellent grip on dry and wet roads, and good road holding for excellent steering precision and stability through bends.