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Decode tire size notation (e.g., P225/45R17) to get sidewall height, overall diameter, circumference, and revolutions per mile.
Reading: P225/45R17
Tire Size Formulas:
Sidewall Height (mm) = Width x Aspect Ratio / 100
Overall Diameter = Rim Diameter + 2 x Sidewall Height
Circumference = pi x Overall Diameter
Width is in millimeters, aspect ratio is a percentage, rim diameter is in inches.
P = passenger tire. 225 = section width in mm. 45 = aspect ratio (sidewall height is 45% of width). R = radial construction. 17 = rim diameter in inches.
Aspect ratio is the ratio of sidewall height to section width, expressed as a percentage. A lower number means a shorter, sportier sidewall.
The overall tire diameter includes the rim. Two tires with the same width but different rim diameters will have very different overall diameters and affect speedometer accuracy.
Your speedometer is calibrated for your stock tire size. A larger overall diameter means the speedometer reads lower than actual speed. Revolutions per mile is the key metric.
Plus-sizing means going to a larger rim diameter while keeping the overall tire diameter nearly the same by reducing the aspect ratio. This improves handling but can reduce ride comfort.