Car
How to read tyre sizes and compare two sizes
Every number on a tyre's sidewall means something.
Decoding 205/55R16 91V
| Marking | Meaning |
|---|---|
| 205 | Section width in millimetres |
| 55 | Aspect ratio: sidewall height is 55% of the width (112.75 mm) |
| R | Radial construction |
| 16 | Rim diameter in inches |
| 91 | Load index: 615 kg per tyre |
| V | Speed rating: up to 240 km/h |
Working out the overall diameter
Diameter = rim × 25.4 + 2 × sidewall height. For 205/55R16 that's 406.4 + 2 × 112.75 = 631.9 mm. For 225/45R17 it's 431.8 + 2 × 101.25 = 634.3 mm, which is 0.38% larger. When the speedometer shows 100 km/h, you'd actually be doing about 100.4 km/h.
💡 Pro Tip: Larger diameters make the speedometer read low, which risks a speeding fine. Smaller diameters make it read high. Keep any change small, and check your state's rules on modifications.
Before you change size
1
Check the tyre placard
It's usually on the driver's door pillar and lists the approved sizes, load ratings and pressures.
2
Match or exceed the load and speed ratings
Never fit tyres rated below the placard.
3
Check clearance
Wider tyres can rub on suspension parts or guards at full lock or full compression.
✨ Key Takeaways
- Diameter = rim × 25.4 + 2 × width × aspect%.
- The load index and speed rating must meet the placard.
- Bigger tyres make the speedometer read low.
Open the tyre size calculator →
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