How to choose an equivalent size
Your speedometer, ABS and ESP calculate distance per wheel revolution from the factory tyre. An equivalent size must therefore keep the overall diameter very close to the factory size. The commonly accepted limit is ±3%; many tyre fitters prefer ±2%.
Overall diameter = 2 × width × (aspect ÷ 100) + rim × 25.4. For 205/55R16: 2 × 205 × 0.55 + 16 × 25.4 = 631.9 mm.
How far off will the speedometer be?
The percentage difference in diameter roughly equals the difference in real speed. With a tyre 3% larger, an indicated 100 km/h is really about 103 km/h. A smaller tyre does the opposite.
Diameter is not the only check
- The load index and speed rating must not be lower than the factory values.
- A wider tyre may rub on the arch liner or suspension.
- The rim width must suit the new tyre width.
- A size that is not on the vehicle's approval may be a problem at roadworthiness inspection. Sizes listed on the certificate of conformity (CoC) are the safest choice.