This example compares two familiar metric radial codes without claiming that the second size fits a particular vehicle. The starting size is 205/55 R16, the comparison is 225/45 R17, and the indicated speed is 100 km/h. Every size-derived decimal is calculated from exact rational inputs; only circumference and revolution outputs require an approximation of pi.
Original diameter is exactly 631.9 mm
The original sidewall is 205 × 55 ÷ 100 = 112.75 mm. The 16-inch rim is 16 × 25.4 = 406.4 mm. Add two sidewalls: 406.4 + 2 × 112.75 = 631.9 mm. Its ideal circumference is approximately 1985.17 mm, but that pi-based value is not used to calculate the speed ratio.
Comparison diameter is exactly 634.3 mm
The comparison sidewall is 225 × 45 ÷ 100 = 101.25 mm. The 17-inch rim is exactly 431.8 mm. Therefore 431.8 + 2 × 101.25 = 634.3 mm. Although the rim is one inch larger, the shorter sidewall keeps the outside diameter close to the original. The nominal increase is 2.4 mm, so radius and ground-clearance change are 1.2 mm.
Diameter change is about +0.3798 percent
The exact percentage is 2.4 ÷ 631.9 × 100 = 2400/6319 percent, displayed as about +0.3798 percent. At an indicated 100 km/h, the nominal ratio gives 100 × 634.3 ÷ 631.9, about 100.3798 km/h. This is a transparent theoretical comparison, not a compatibility verdict or a prediction of the vehicle’s measured GPS difference.