Tyre size

There are three things to understand when buying new tyres for your car; the size, speed rating and load index. Making the correct selection is very important to maintain optimal performance, safety and fuel efficiency.

Fortunately, all of this information is easy to find. You can locate it inside the driver’s door on your car, inside the fuel flap or inside the glove compartment. Alternatively, you can consult your vehicle documentation and tyre manufacturer specifications. 

Reading tyre size documentation

What do all these letters and numbers mean? Here’s all the information on the tyre sidewall quickly decoded:

• 1)    The width of the tyres in millimetres
• 2)    Percentage height of the sidewall in relation to the width
• 3)    Radial construction
• 4)    Rim diameter in inches
• 5)    Load index (the maximum load capacity of the tyre)
• 6)    Speed index (the maximum speed of the tyre)
• 7)    Self-Supporting Run-flat tyre)
• 8)    Mercedes Original Equipment key (serves as an example: find more OE keys here)
• 9)    Suitable for mud and snow conditions
• 10)    Compliant with Department for Transport vehicle safety standards
• 11)    Week of manufacture
• 12)    Year of manufacture

The documentation supplied with your vehicle outlines the recommended tyre sizes for your car. Compatibility is determined according to its outer dimensions – diameter/rolling circumference and width – and labelled as a standardised tyre size indicator on the sidewall.

Also, the size must comply with the vehicle’s requirements regarding speed rating and load range. Together, the load index and speed index make up the full service description for a passenger car tyre. It must correspond to the information provided in the vehicle documents.

Load index

The load index rating is the maximum capacity of a passenger car tyre, calculated from the maximum permissible axle load range between two tyres.

Speed index

The speed index rating is also a relevant factor when choosing a tyre. The tyre’s speed capability must be at least equivalent to the top speed of the vehicle. Its speed rating indicates the maximum permissible speed (at full load) of a tyre, and whether it’s appropriate for your car.

Choosing the correct tyre size

Make your tyre size selection in strict accordance with vehicle documents and tyre manufacturer recommendations. If the combined wheel and tyre diameter aren’t correctly matched, there may be drastic differences in ride height and speedometer readings.

For example, fitting 17-inch-diameter wheels to a car that is designed to have 14-inch-diameter wheels will change the ride height. That means the car body will be higher off the ground. There will also be a slight increase in top speed, and the speedometer will no longer be accurate. Last but not least, the car will accelerate slower.

Switching to different tyre sizes can only be done in compliance with all legal requirements and regulations – plus the recommendations of the vehicle, wheel and tyre manufacturers. At the very least, the wheel must have complete freedom of circular motion and the tyre’s adequate load capacity must be observed.

The internal construction of tyres fitted on any vehicle must either be radial or bias-ply. Fitting mixed tyre constructions (bias-ply and radial together) for cars, caravans and other light trucks is unsafe and illegal. The only permissible exception is the use of a spare tyre for an emergency.

Concerning the choice of wheels or rims, the same guidelines apply. Motorists must use the standard wheels or rims approved by the vehicle manufacturer as recommended.

Replacing fewer than four tyres

For optimal safety and performance, replace all four tyres at the same time. While it is possible to switch out less than four tyres at the same time, you should follow a couple of guidelines regarding tyre size.

If only replacing one or two tyres, for example, ensure that each one is the same size and has the same load index and speed rating as specified by the car manufacturer.

When replacing only two tyres, fit both of them to the wheels on the rear axle. The reason for this is that the newer tyres will have much better grip, particularly on wet roads, and reduce the risks of aquaplaning.

Replacing only a single tyre is not advisable, as it can impact vehicle suspension or transmission and produce excessive wear on the tyre tread. But if replacing only one tyre is unavoidable, then pair the single replacement tyre with the tyre that has the deepest tread depth, then fit both to the rear axle.