Knowing some basic information about tire 
dimensions
and sizes, along with a few technical aspects of tire construction, will help clear up much of the confusion many people have in understanding tires and tire sizes.
There are up to 14 parts to a tire and each one is subjected to different stresses and performance requirements. About 89% of a tire is rubber compound. The remainder consists of 8.5% ply and belt cord materials, and 2.5% steel wire used in the bead.
Understanding the sizing of tires can be especially confusing. There are two popular methods of identifying lawn and garden tires. The first is (overall diameter) x (section width) - (rim diameter). An example is a 16x650-8 tire.
The second popular method of designating tire size includes (section width) / (section height) - (rim diameter). For example a 4.10/3.50-4. The section width and section height are also used to size passenger car tires. For instance, a p205/75r14 tire is explained in this manner: P stands for passenger car tire; the 205 indicates the section width in millimeters; the 75 indicates the section height to width ratio (%); the R stands for radial construction; and the 14 is the rim diameter in inches.
Tire Terms and Definitions
Following are some definitions of terms used in tire descriptions:
BEAD: The inner portion of the tire which fits onto the rim and contains bands of wire to which plies are attached.
PLY RATING: Index of tire strength; Indicates maximum recommended load for specific types of service. It usually does not indicate actual number of cord plies in the tire as it did in the past.
RIM DIAMETER: The distance between the vertical portion of the rim flange. If the rim is too wide or too narrow, the tire bead will not seat properly. As a general rule, the proper rim width for a tire will be 1/2 to 1 1/2 inches narrower the the section width of the tire, depending on the size of the tire.
SECTION WIDTH: The distance across the tire at its widest part when inflated but not under a load (does not include protective bars or decorations).
SECTION HEIGHT: The distance between tread crown and bead seat when the tire is inflated but not under a load.
The Tire Rim Association has set an acceptable variance, of 8% from their published dimensions for size tire designations. Because of this variance it is always wise to be aware of the possibility of tread and dimensional differences when replacing tires. This becomes especially important when replacing tires on equipment where the tire dimensions are critical to equipment performance.
