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How the speedometer comparison is calculated
A circumference ratio estimates one effect. It does not measure the vehicle’s actual speed.
Reviewed Oct 27, 2026
Start with the ratio
TireSpan uses indicated speed × new circumference ÷ original circumference. In its circular geometry model, circumference is proportional to diameter, so the same calculation can use the diameter ratio. The Original input defines the calibration reference for this model.
Calculate the reference row
For 215/55R17 to 225/45R18, the ratio is 659.7 ÷ 668.3 = 0.9871315… . Multiply an indicated 100 km/h by that ratio to get 98.71315… km/h, displayed as 98.71 km/h. At an indicated 50 km/h, the theoretical value is 49.36 km/h after rounding.
Swapping requires a new calculation
With 225/45R18 as original and 215/55R17 as new, the ratio becomes 668.3 ÷ 659.7 = 1.0130362… . At 100 km/h indicated, the result is 101.30 km/h. That increase is not exactly the earlier decrease with its sign reversed because the denominator changed.
Keep the calibration assumption visible
The estimate assumes that the original calibration corresponds to the original nominal circumference. It does not know the instrument’s starting error or the rolling circumference of either tire in service. Those unknowns are why the result is labeled theoretical, including in downloads.
Choose rows that explain the comparison
Use several indicated speeds if a table makes the ratio easier to understand. The rows are examples of the same constant-ratio model, not independent measurements. A zero indicated speed remains zero, and comparing identical sizes preserves every indicated speed without implying that the instrument itself is accurate.