There are 2 methods used to measure tires:
1)If an "x" separates the first 2 sets of numbers, the first number is the overall diameter of the tire. The second number is the overall width. The last number represents the tire diameter.
2)Method 2 is used for the smaller and industrial tires. If a "/" separates the first 2 sets of numbers the first number is the overall width of the tire. The second number identifies the section height. This is the measurement from the crown to the bead of the tire. The last number is the rim diameter. The overall diameter of the tire is D + D + C. so for a 4.10/3.50-4 the overall diameter is 11" (3.5 + 3.5 + 4).
Note: All measurements should be made when the tire is inflated to its recommended air pressure.