Tyre size calculator
Diameter and circumference of a tyre (e.g. 205/55 R16), and the speedometer error if you change size.
How it works
Diameter = rim × 25.4 + 2 × width × aspect / 100. The diameter difference shouldn't exceed ± 3% (beyond that, the speedometer and driver aids are off).
Results are for guidance only and do not replace professional advice.
Worked example
With the default values:
- Current tyre: width (mm)205
- Current tyre: aspect ratio (%)55
- Current tyre: rim (inches)16
- New tyre: width (mm)225
- New tyre: aspect ratio (%)45
- New tyre: rim (inches)17
you get:
- Diameter difference+0.38 % ✓
- Current / new diameter632 mm → 634 mm
- Current / new circumference198.5 cm → 199.3 cm
- At an indicated 100 km/h you actually drive100.4 km/h
Frequently asked questions
How does the “Tyre size calculator” tool work?
Diameter = rim × 25.4 + 2 × width × aspect / 100. The diameter difference shouldn't exceed ± 3% (beyond that, the speedometer and driver aids are off).
What do I need to enter?
Just fill in: current tyre: width, current tyre: aspect ratio, current tyre: rim, new tyre: width, new tyre: aspect ratio, new tyre: rim. The result updates as you type.
Can you give an example?
Current tyre: width (mm): 205; Current tyre: aspect ratio (%): 55; Current tyre: rim (inches): 16; New tyre: width (mm): 225; New tyre: aspect ratio (%): 45 → Diameter difference: +0.38 % ✓; Current / new diameter: 632 mm → 634 mm.
Is this calculator free, and is my data private?
Yes. HOD Calc is completely free with no sign-up. The calculation runs in your browser: nothing you type is sent or stored.