Why rolling circumference can differ

The calculation is a circumference of a circle

TireSpan multiplies nominal diameter by π. For 225/45R18, 659.7 × π gives about 2,072.51 mm. Dividing a kilometer or a mile by that length gives a geometric revolution count. The arithmetic is deterministic and uses no road-test data.

Rolling circumference refers to motion

Distance traveled per revolution describes a tire rolling under specified conditions. A loaded pneumatic tire deforms, and its operating state differs from a perfectly rigid circle. Tire model, loading, inflation, wear and test conditions belong with any real rolling measurement. TireSpan does not accept those as inputs, so it cannot correct for them.

Do not assume every radius is the same radius

The unloaded outer radius, the distance from hub center to the ground under load, and an effective rolling radius describe different measurements. Multiplying a loaded vertical radius by 2π is not automatically a valid way to obtain distance traveled per revolution. Use the particular definition attached to the measurement.

What that means for the speed table

The speed table answers a controlled question: what is the ratio implied by two nominal circular circumferences? It holds the rest of the calibration assumption constant. It does not read a vehicle’s sensors or prove the actual speed at a displayed reading. The report labels the result theoretical so the assumption travels with the number.

Keep measurements with their conditions

When working with a product data sheet, note the tire model and the manufacturer’s stated conditions. When documenting a nominal comparison, keep the original and new markings together. Mixing a measured revolution count for one tire with a calculated diameter for the other changes the method and should never happen silently.

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