Docs
Use the theoretical speed table
The original size defines the reference for every row.
Reviewed Oct 28, 2026
Enter the indicated speeds you want to compare
Use the speed controls to choose 1–12 indicated-speed examples. Each value must be between 0 and 400, inclusive, in the currently selected speed unit. Each row uses the same nominal circumference ratio. These input limits are calculation bounds, not permitted or recommended driving speeds.
Correct invalid or unsupported values before exporting. A speed row is a hypothetical calculation, not a recorded observation. Read the unit with the row so a value entered in mph is not mistaken for km/h.
Follow one row through the formula
Theoretical speed equals indicated speed multiplied by new circumference divided by original circumference. For 215/55R17 to 225/45R18, 100 × 659.7 ÷ 668.3 gives 98.71 km/h after display rounding. The default rows provide more examples of that same ratio.
Recheck rows after swapping
When you swap the tire inputs, the original denominator changes. The table therefore recalculates. For the reversed default pair, an indicated 100 km/h gives a theoretical 101.30 km/h. It is not a simple sign change applied to the earlier displayed number.
Keep “theoretical” attached to the result
TireSpan does not measure wheel rotation, road speed or instrument calibration. It assumes the original calibration corresponds to the original nominal circumference and isolates the ratio change. Actual rolling circumference and existing instrument bias are outside the model.
Check the selected rows in the report
CSV and PDF outputs use the current valid result and selected speed examples. Recalculate after changing inputs, then export a new file. A saved report cannot update itself when the browser comparison changes.