Tyre size conversion: calculating compatible sizes for your vehicle
When changing wheel and tyre size, it’s important to ensure the compatibility of the tyre diameter, speed rating and load index of the tyres you wish to fit. You need to ensure that:
The outer tyre diameter remains as close as possible (within a margin of +/- 3%) to that of the original tyre to maintain the accuracy of the speedometer and driving aid settings (ABS, ESP etc.).
- The load index is the same as or higher than that of the original tyre.
- The speed rating is the same as or higher than that of the original tyre, (except if you’re replacing a summer tyre with a winter tyre which may have lower speeds due to winter weather conditions).
It’s also important to take into account your vehicle’s specific technical characteristics. There is no guarantee that a size which is compatible in theory will fit your vehicle in reality. For example, you may find yourself with a tyre you cannot fit because it touches the suspension.
Therefore, if you to want to fit a non manufacturer-approved size, you should also:
- Check the selected tyre size is compatible with the structure of your vehicle
- Maintain the required clearance at the wheel arches (statically and dynamically)
- Check there is space for the brake callipers
- Check the wheel fitment and offsets.