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Tyre geometry & unit converter
Pure geometry and SI conversions. Useful at the tyre machine and on the counter. Not a fitment guarantee and not a TPMS relearn procedure.
Metric size & speedo error
Stock diameter
634.3 mm
Sidewall 101.3 mm
Fitted diameter
645.2 mm
+1.72% vs stock
True speed at indicated
101.7 km/h
When the cluster shows 100
Offset ↔ backspace
Units
Pound-feet
81.1 lbf·ft
Back to Nm
110 Nm
Kilopascals
220.6 kPa
Back to PSI
32 PSI
°F
68 °F
Back to °C
20 °C
Theory
Diameter changes indicated speed
Overall diameter = 2 × section × (aspect/100) + rim inches × 25.4. A taller tyre makes the cluster read slow; a shorter tyre reads fast. Plus-sizing that changes diameter or offset can move load index, speedo error, and — after an alignment — ADAS calibration. That last step is an I-CAR question, not a geometry question.
How to use it
- Role
- Tyre fitter / advisor (P10 / P02)
- Shop-day step
- ESTIMATE → PARTS
- Do
- Compare stock vs fitted diameter, check load index kg against the plate, quote alignment + cal when the size or thrust line changes.
- Do not
- Do not guarantee fitment or skip TPMS relearn because the calculator looked fine.
Official sources
Related tools
Geometry only. Load index is ETRTO typical kg — confirm the sidewall and the vehicle plate.