Calculator
Tyre size comparison calculator
Compare geometric dimensions encoded in tyre sizes. This tool does not check rim width, offset, bolt pattern, load capacity or vehicle approval. Even a small diameter difference does not confirm a suitable fitment.
Calculator
Enter data
Width, aspect ratio and rim diameter for two sizes, plus indicated speed. Fields marked with an asterisk are required.
How to use the calculator
- Enter both sidewall sizes, for example 205/55 R16.
- Before changing sizes check the vehicle manufacturer’s approvals and consult a tyre professional.
Calculation details
Formula and calculation method
Diameter D = 2 × width × aspect/100 + inches × 25.4. Circumference = πD; nominal clearance changes by (D2 − D1)/2. Modelled speed = indication × D2/D1.
Worked example
205/55 R16 gives 631.9 mm; 225/45 R17 gives 634.3 mm. The difference is about +0.38%, with a modelled 100.4 km/h at a 100 km/h indication.
Limitations
- The model assumes an ideal indication for the old size. Factory speedometer error, load deflection, pressure, wear and actual rolling circumference are excluded.
- No safety verdict or universal allowable difference. Nominal geometry does not establish vehicle approval.
Sources
Engine version: 2.0 (2026-09-23) · Last reviewed: 23 September 2026
Frequently asked questions
Does this select rims?
No. It checks nominal tyre geometry and bead-seat diameter, not offset, width or mounting.
Is the resulting speed the actual speed?
No. It is a ratio comparison assuming an ideal speedometer for the previous tyre.