Convert the three dimensions consistently
For a size such as 225/45R17, the first number supplies nominal section width in millimeters, the second supplies sidewall height as a percentage of that width, and the last supplies rim diameter in inches. The model calculates sidewall height as width times aspect ratio, then adds two sidewalls to the rim diameter converted using 25.4 mm per inch.
The resulting circle is an unloaded nominal model. It is useful for comparing codes and displaying the arithmetic, but it is not a tire manufacturer’s measured overall diameter or the loaded rolling radius. Construction, measuring rim, pressure, wear and load can affect real dimensions.
Apply the ratio in the correct direction
For the same indicated rotational speed, a larger nominal circumference predicts more travel per revolution. The comparison therefore multiplies indicated speed by candidate diameter divided by baseline diameter. The inverse value answers a different question: what indication would correspond to the same nominal road speed after the change?
The calculation assumes the baseline indication was exact. It does not measure an existing speedometer error, tire slip or electronics calibration. The same ratio applies whether the selected unit is km/h or mph; changing the display unit does not convert an entered number automatically.
Do not turn a percentage into an approval rule
The report displays the diameter percentage without calling a particular percentage universally safe or legal. Vehicle systems and manufacturer requirements can depend on more than overall diameter, including differences between axles and the tire’s service description.
Keep the numeric comparison with the equipment specifications you are discussing. Check the vehicle and tire manufacturer’s requirements separately before changing size. This tool provides a transparent measurement record rather than a recommended replacement list.