What can a car diagnostic test tell you?

Aspect Ratio

The fourth and fifth digits of the tyre code that immediately follow the tyre width. The aspect ratio or profile height of the tyre sidewall is expressed as a percentage of the tyre width. So an aspect ratio of 55 for example means that the profile height of the tyre is 55% of its width.

Rim Diameter

The next two digits represent the size of the wheel rim that the tyre can be fitted to. It is also the diameter of the tyre from bead to bead.  So a tyre marked 16 will fit on a 16-inch wheel rim.

Speed Rating

The speed rating of a tyre is represented by a letter of the alphabet at the end of the tyre size code and indicates the maximum speed capability of the tyre. Tyres receive a speed rating based on a series of tests which measure the tyres capability to handle a set speed for a prolonged period of time.

Select your tyre speed rating to find out the maximum speed your tyres can maintain.

If you are unsure what speed rating you need, be sure to check your vehicle handbook. Choosing a lower speed rating than that recommended by your vehicle manufacturer could potentially invalidate your insurance.

Load Rating

The load index provides information on the maximum weight capability for the tyre. The load index is a numerical code that can be located just after the tyre size marking and before the speed rating.

Select your load index to find out the maximum load capability for your tyres.

Although not illegal, it is not advisable to have tyres with a lower speed rating or load index than the manufacturer recommended tyre specification for your vehicle, or to have a combination of different tyre construction types. Consult your vehicle handbook to confirm your vehicle’s tyre speed rating and load index as well as any additional requirements.