225/50 r18

225/50 R18 is a tire size specification:

- 225: tire width in millimeters
- 50: aspect ratio; sidewall height is 50% of the width
- R: radial construction
- 18: wheel/rim diameter in inches

So it means a radial tire 225 mm wide, with a sidewall height equal to 50% of 225 mm, made to fit an 18-inch rim.