How a different tire size changes speedometer and distance readings
Compare tire diameters before changing wheel or tire size, and understand the speedometer estimate without confusing it with an approval.
How to calculate it
Read the complete sidewall size: width in millimetres, aspect ratio as a percentage of width, and wheel diameter in inches. Sidewall height is width × aspect ratio ÷ 100. Overall unloaded diameter is wheel diameter × 25.4 plus twice sidewall height, in millimetres. A circumference estimate is diameter × π. Compare new circumference with the original; the ratio describes the approximate change in revolutions per distance.
Worked example
For a 205/55R16 tire, sidewall height is 112.75 mm and diameter is 406.4 + 225.5 = 631.9 mm. For 225/45R17, height is 101.25 mm and diameter is 431.8 + 202.5 = 634.3 mm. The new diameter is about 0.38% larger. If a speedometer were calibrated exactly to the old rolling circumference and showed 100 km/h, the simple ratio suggests about 100.38 km/h actual, before its built-in calibration offset.
What can change the result
A loaded tire deforms, wears down and changes with pressure. Actual rolling circumference is not exactly π times an unloaded diameter. Vehicle tolerances, brake clearance, load index, speed rating and legal requirements matter. A small numeric difference does not certify fitment or instrument accuracy. Consult the vehicle maker and a qualified installer before changing sizes.
Use the calculators and keep reading
Enter both complete sizes into the tire comparison calculator and inspect diameter and circumference together. Check the related tire pressure guide for cold inflation and do not choose a size using the speedometer ratio alone.
Try the tire comparison, tire diameter calculator and read this related article for the context and assumptions.