Static overall diameter from section width, aspect, and rim. Rolling diameter is not this number.
Size decoder
Passenger example: P225/40R18 92Y.
Front (vehicle)
Ø 0.643 m (25.33 in)
Rear (vehicle / speedo stock)
Ø 0.643 m (25.33 in)
Compare size (speedo)
Results
At a true 30 km/h, the speedo would read about 29.5 km/h with the compare tire. Rolling diameter under load is smaller than this static figure — typically 2–4%.