tyre rolling diameter Page 182

“Rolling diameter” is the effective overall diameter your tyre presents when actually moving under load, not just what’s on the sidewall.

To estimate from a size like 225/50R17: Sidewall = width × aspect = 225 × 0.50 = 112.5 mm; Overall diameter ≈ 2 × sidewall + rim (in mm) = 2 × 112.5 + 17 × 25.4 ≈ 657 mm; Circumference ≈ π × diameter ≈ 2.06 m.

Real rolling diameter is typically 1–3% smaller thanks to tyre deflection, which is why manufacturers quote revs per kilometre.

When upsizing, keep overall diameter within about ±2–3% of OEM to keep the speedo, ABS and gearing happy.

If you need precision, measure revolutions per kilometre on the spec sheet or a GPS run, then back‑calc diameter.

Consult official documentation for exact figures.