How the numbers are worked out
Sidewall height is the width multiplied by the aspect ratio. Overall diameter is the rim size plus two sidewalls, converted to inches. A bigger diameter means each turn of the wheel covers more ground, so your speedometer reads low.
sidewall (mm) = width × aspect ÷ 100
diameter (in) = rim + 2 × sidewall ÷ 25.4
actual speed = indicated × new diameter ÷ old diameter
Frequently asked questions
225 is the tread width in millimetres, 45 means the sidewall is 45% as tall as the tire is wide, R means radial construction and 17 is the wheel diameter in inches.
Many shops aim to stay within about 3% of the original diameter so the speedometer, ABS and stability systems behave as designed. Always confirm clearance and load rating for your car.