Cars & Vehicles
Tire size calculator
Width / aspect / rim → diameter, circumference and revs per km.
- Instant
- Free
- Private (processed locally)
- No sign-up
Decode a tire marking
Enter the width, the aspect ratio and the rim diameter. The tool computes the overall diameter, circumference, sidewall height and revs per km.
The formulas
- Sidewall = width × aspect ÷ 100
- Diameter = rim × 25.4 + 2 × sidewall
- Circumference = π × diameter
- Revs/km = 1,000,000 ÷ circumference (mm)
Example (205/55 R16)
| Input | Value |
|---|---|
| Sidewall | 112.75 mm |
| Diameter | ≈ 632 mm |
| Circumference | ≈ 1985 mm |
| Revs / km | ≈ 504 |
100% local calculation, no data sent.
Frequently asked questions
How do I read 205/55 R16?
205 = width in mm, 55 = aspect ratio (sidewall height as % of width), R = radial, 16 = rim diameter in inches.
How do I calculate the diameter?
Diameter = rim × 25.4 + 2 × (width × aspect ÷ 100). For 205/55 R16: 16 × 25.4 + 2 × 112.75 ≈ 632 mm.
Why compare revolutions per km?
Two tires of different diameters spin at different speeds: a gap throws off the speedometer and odometer.
What gap is acceptable?
People usually aim for under 3% diameter difference vs the original size to keep the speedometer and clearance right.