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Millimeters, inches and tire dimensions
The marking mixes units. The calculation should not mix assumptions.
Reviewed Oct 27, 2026
A metric marking still contains inches
In 225/45R18, width is given in millimeters, aspect ratio is a percentage and rim diameter is given in inches. Switching the display to inches does not change the meaning of the marking. It changes how lengths are presented after calculation.
Use one exact conversion
One inch is exactly 25.4 mm. An 18-inch nominal rim diameter is therefore 457.2 mm. A 659.7 mm nominal tire diameter is 659.7 ÷ 25.4 inches, approximately 25.97 inches when shown to two decimal places. Convert from the full-precision value, not from a rounded value copied off a screen.
Display precision is not input precision
Rounding makes a table readable. It should not become a new calculation step. If a report shows a rounded circumference and a rounded revolution count, recomputing one from the other may differ in its last displayed digit. The original model retains more precision than either displayed value.
Keep speed units consistent within a row
A speed ratio is dimensionless, so it can be applied to km/h or mph. Converting between the two uses 1 mile = 1.609344 km. Do not compare a new mph value with an original km/h value and call their numeric difference a speed change.
Read units with every exported number
The CSV includes stable numeric columns and explicit units. A spreadsheet’s locale may affect how it displays numbers, but the file’s machine representation uses decimal points. The PDF and SVG prioritize readable labels. Keep the JSON project when you need to recover the editable input values rather than transcribing a formatted report.