Diameter and circumference from tire spec (e.g. 225/45R17).
Parses a tire spec like 225/45R17 into diameter and circumference.
225 = section width mm, 45 = aspect ratio (%), R = radial, 17 = wheel diameter inches.
Circumference matters beyond speedometer accuracy — it also affects gearing feel and, on all-wheel-drive vehicles, mismatched tire circumferences across axles can strain the drivetrain, which is why tire replacement guidance is stricter for AWD cars.
225/45R17: overall diameter 634.3 mm (24.97 in), circumference 1,992.7 mm.
Bigger diameter → speedo reads slower than actual speed. Changing tire size changes odometer readings too.
Numbers after size (e.g. 91V) indicate max load and speed. Replace only with equal or higher rated tires.
Generally not recommended, and often unsafe on AWD vehicles specifically — even a small circumference difference between tires can cause drivetrain wear over time.