To the untrained eye, looking at all the numbers and gibberish on the side of tire must seem like looking at hieroglyphics. But armed with a little knowledge, they're easy to figure out. There are three main sets of numbers you need to concern yourself with on the side of a tire; the DOT number, the size, and the service description.
The DOT, or Department of Transportation, number is used by the manufacturer to identify the specific tire, where and in what batch it was manufactured, and its manufacture date. This number is found near the bead of the tire and ends with four numbers. These numbers designate the manufacture date. For example, if the last four digits are 1401, the tire was made in the 14th week of 2001. The last two digits refer to the year and the first two refer to the week of the year that the tire was constructed.
The tire size is usually displayed higher up on the sidewall. It contains a wealth of information about the tire. In most modern vehicles it's displayed in a format similar to this: P205/55HR16. These numbers and letters vary between tires sizes, but here's what they mean. The P stands for P-metric, meaning that the tire is meant for a passenger car. The 205 refers to the tire width in millimeters. The 55 is the aspect ratio. This refers to the height of the tire from the top of the rim to the top of the tire. In this case, the tire is 55% as tall as it is wide. The H refers to the speed rating, or the maximum sustained speed a tire can travel at. There are several different speed ratings. The 16 is the rim diameter in inches.
On most tires, you'll see the tire size followed by a 97H or something similar. The 97 is the service load, the maximum amount of weight a tire can hold at its maximum pressure. the 97 references a weight on the service load chart. The higher the number, the more weight a tire can hold. As mentioned above, the H is the speed rating, the maximum sustained speed a tire can safely travel at.
The last thing you need to know is the service description. This lists the maximum amount of weight in pounds and kilograms that a tire can carry at its maximum pressure, which is measured in p.s.i. and kPa. Essentially, this restates the service load in layman's terms.
Armed with this information, deciphering all the numbers on the side of your tires should be easy. Be sure to check your vehicle's owner's manual before purchasing tires for your vehicle.