The difference between 235/60 tires and 225/60 tires is their physical dimensions. A 235/60 tire is 235 millimeters wide, while a 225/60 tire is 225 millimeters. This measurement is from sidewall to sidewall when the tire is on its rim.
Though these tires both have an aspect ratio of 60, their heights are different. The aspect ratio is a percentage that relates the tire's height, measured from the edge that sits on the wheel to the top of the tread, to its width. Thus, the tire that is 235 millimeters wide is 141 (235 multiplied by 60 percent) millimeters high and the tire that is 225 millimeters wide is 135 (225 multiplied by 60 percent) millimeters high.