Comparing two sizes without guessing fitment

Record the starting point

Put the original marking in the Original field and the proposed marking in New. Check the parsed width, aspect ratio and rim diameter before calculating. Keep the complete tire specification separately if you are discussing an actual product; the three dimensions do not preserve every marking on its sidewall.

Read all four dimensions

Look at width, sidewall, rim and overall diameter separately. Similar overall diameters can conceal a different rim diameter, and matching rim diameters do not mean matching widths. The default 17-to-18-inch example shows both: a larger rim, a shorter sidewall and a slightly smaller overall tire.

Avoid a universal percentage shortcut

A percentage describes the size of a change relative to your original. It does not encode load rating, wheel compatibility, clearance or legal requirements. TireSpan does not turn an arbitrary diameter band into a green “safe” badge. A zero difference is also just a zero geometric difference.

Use the report to frame specific questions

Take a report that includes both inputs and the method. It can support a focused conversation about the nominal dimension change. It cannot replace the vehicle manufacturer’s requirements, the wheel and tire specifications, or a check of the actual assembly under relevant conditions.

Keep alternatives identifiable

If you compare several candidates, save descriptive names and retain a JSON backup. Recheck which size is the reference before comparing percentages across reports. A report for A to B and one for B to A answer different reference questions even though they contain the same pair.

Keep exploring