1 - If an "x" separates the 1st set of numbers, the 1st number is the outside diameter on all scooter tyres and the 2nd number is the overall width. If a third number is used, that number represents the inside tyre diameter, or the outside diameter of the rim.
2 - If a "/" separates the 1st set of numbers, the 1st number is the overall width of the tyre and the 2nd number identifies the section height. This is the measurement from the crown to the bead of the tyre. The last number is the rim diameter. The overall diameter of the tyre is D + D + C. For example, a 4.10/3.50-4 tyre's overall diameter is 11" (3.5 + 3.5 + 4). Some sizes omit the tyre width measurement completely, such as a 3.00-4 tyre. In this example, the "3.00" is the measure of the section height and the "4" is the rim diameter, or the inside diameter of the tyre.