For example the tires on your unit may be marked with a designation of standard trailer (ST) 225/75R15 or light truck (LT) 235/85R16. The designation breakdown is as follows:

• The first three-digit number gives the width in millimeters of the tire from sidewall edge to sidewall edge. In general, the larger the number, the wider the tire. 
• The next two-digit number after the "slash" mark, known as the aspect ratio, gives the tire's ratio of the height to width.
• R - the "R" stands for radial.
• The last two-digit number is the wheel or rim diameter in inches.