Complex sentence: The first number in the code "225" represents the nominal tire width in millimeters, this is followed by the aspect ratio "70" which is the height of the side wall expressed as a percentage of the nominal width.
Split into simpler sentences: The first number in the code (e.g., "225") represents the nominal tire width in millimeters. This is followed by the aspect ratio (e.g., "70"), which is the height of the side wall expressed as a percentage of the nominal width.