A speedometer scenario starts with a simple model: the vehicle interprets wheel rotations using calibration associated with the original rolling geometry. If the nominal comparison tire has a different diameter, one rotation represents a different theoretical distance. TireCalc reports that ratio as an estimate, not as the vehicle’s measured error, because real instruments and loaded tires add effects absent from the size code.
Calculate each nominal diameter
Sidewall height equals width times aspect ratio divided by 100. Outside diameter equals rim inches times 25.4 mm plus two sidewalls. For 205/55 R16 the result is 631.9 mm; for 225/45 R17 it is 634.3 mm. Both values remain exact decimals because every input and the inch conversion are rational.
Use the diameter ratio; pi cancels
Circumference is diameter times pi. When comparison circumference is divided by original circumference, pi appears above and below and cancels. The theoretical actual speed is therefore indicated speed × comparison diameter ÷ original diameter. At an indicated 100 km/h, the reviewed pair gives about 100.3798 km/h. The calculator never feeds a rounded circumference into this exact ratio.
Treat the output as nominal geometry
Actual speed can differ because the original speedometer may already have intentional or permitted calibration bias, and because rolling circumference changes with pressure, load, tread depth, wear, temperature, construction, and speed. A diameter percentage is not a universal legal tolerance. It does not check load rating, speed rating, rim width, wheel offset, brakes, suspension, body clearance, chains, or electronic-system calibration.