The '225' indicates the tire's width in millimeters. The '75' is the aspect ratio, signifying that the height is 75% of the width. To calculate the height: 225mm x 0.75 = 168.75mm. Convert this to inches (168.75 ÷ 25.4) to get approximately 6.65 inches. Remember, this is just the sidewall height. The overall diameter includes the rim, so add 15 inches (from the 'R15') twice: 6.65 + 6.65 + 15 = 28.3 inches in total diameter.