Tip: You can find your tyre size on the sidewall of your current tyre, in your vehicle’s handbook or on a sticker in the door jam or fuel cap.
Understanding Tyre Size
This is the overall width of the tyre, represented by the first 3 digits on the sidewall of the tyre. Wider tyres offer better grip, but can create excessive amounts of road noise.
The 'aspect ratio' of the tyre is the height of the tyre’s sidewall divided by the width of the tyre, multiplied by 100. In our example, this means that the overall tyre profile is 55% the size of the tyre’s width.
If you removed a wheel’s tyre, you would be left with the wheel rim. The width of the rim may vary slightly from the width of the tyre, which is why there the two dimensions are given separately.
The speed rating is a letter which indicates the maximum speed at which the tyre can carry the load corresponding to the load capacity index. In our example, a “V” rating can cope with speeds of up to 148 mph.
The load capacity index represents the maximum weight the tyre can carry at the speed indicated by the speed rating symbol. In our example, the tyre has a load index of 91, meaning it can support up to 615kg.
Extra Load (XL)
Some vehicles require tyres that are rated to carry a higher load and therefore have a higher inflation pressure. This information can be found on the tyre sidewall marking and is marked as Extra Load (XL) or Reinforced (RF).
Runflat tyres are made in reinforced sidewalls which carry the weight of the vehicle in the event of pressure loss. This ‘Self-Supporting’ Tyre Technology is typically foudn on BMWs and Minis and enables you to continue your journey with a flat tyre for up to 50 miles at up to 50 miles an hour.
What is my tyre size?
You can find your car’s tyre size written on the sidewall of
your current set of tyres, in your vehicle’s handbook or on a sticker in the
door jam or inside the fuel cap door. When buying new tyres you should only go
for ones that have these same numbers to ensure a good fit.
1. This number is the tyre’s width in mm.
2. This number is the tyre’s profile/ aspect ratio (height
of the sidewall)
3. This number is the wheel rim’s diameter (the distance
across the wheel) in inches
4. ‘R’ means the tyre has a radial design
5. This number corresponds to a tyre’s load index, which
shows how much weight your car’s tyres can handle.
6. This letter is a tyre’s speed rating, indicating the
highest speeds the tyre can handle.