The first numbers indicate the width of the tyre in inches (1 inch = 25.4 mm), or respectively 12.4 inches (12.4 inches x 25.4 mm = 310 mm in width) and 10.5 inches (10.5 inches x 25.4 mm = 260 mm in width). The last numbers indicate the rim diameter, always displayed in inches, in this case, 28 inches and 16 inches.