What changes when tire diameter changes

Circumference follows diameter in the same proportion

In the circular model C = πD. Multiply diameter by a ratio and circumference changes by that same ratio. A nominal diameter change of −1.29% therefore corresponds to the same percentage change in nominal circumference. The absolute units differ: one is a straight-line length and the other goes around the circle.

Revolutions move in the opposite direction

For a fixed distance, a smaller circumference requires more turns. The relationship is reciprocal. In the default comparison, revolutions per kilometer rise from about 476.30 to 482.51. The percentage increase is not obtained by merely reversing the sign of the rounded diameter percentage.

The theoretical speed row uses the circumference ratio

At a fixed indicated speed under the stated original-size calibration assumption, the smaller new circumference yields a smaller theoretical speed. For 215/55R17 to 225/45R18, an indicated 100 km/h becomes a theoretical 98.71 km/h. No sensor reading is involved.

A radius change is half the diameter change

The default new tire’s nominal diameter is 8.6 mm smaller, so its nominal radius is 4.3 mm smaller. This is why common-ground alignment shows centers separated by 4.3 mm. The illustration does not calculate loaded ride height, suspension travel or clearance to the body.

Some effects cannot be inferred from diameter alone

The same diameter can come from different widths, sidewall heights and rim diameters. A geometry table does not provide grip, comfort, load capacity, braking distance or drivetrain compatibility. Use the connected arithmetic to understand the model, then obtain the separate information needed for any physical change.

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