Circunferência do Pneu

Tire circumference: the formula and why it matters

Circumference is C = π × D, where D = 2 × width × profile + rim × 25.4 (all in mm). Take a 205/55 R16: D = 2 × 205 × 0.55 + 16 × 25.4 = 225.5 + 406.4 ≈ 631 mm, so C lands around 1,982 mm. A given circumference covers the same ground on every turn of the wheel, and that's exactly what the speedometer reads off of, through V_actual = V_indicated × (new_diameter / original_diameter). If you go up in rim size, say from 16" to 18", trim the profile so C stays near the factory figure. Skip that and your speedometer and ABS end up reading wrong.

Applications

Going up a rim size on a tuned car. Off-road builds running 35" or 37" tires. Getting the speedometer back in line after a tire swap. Keeping the odometer honest on a fleet. Working out rotation frequency for ABS and traction control.

FAQ

How much can I change tire diameter without recalibrating? Stay under roughly 3% and the speedometer error keeps within legal tolerance. Past that, you'll want to recalibrate the ECU or fit a corrector module.

Does width affect circumference? It does. Sidewall height comes out to width × profile, so a wider tire kept at the same profile percentage ends up taller too.

Why is my GPS slower than the speedometer? Carmakers build in a positive bias, usually 3-5%, as a safety margin. GPS tracks real speed over the ground, so redoing the math with your actual tire circumference tells you which reading to trust.