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.