by Biloxi Von Lutz

Created on: May 11, 2010

Take a look at the sidewall of the tires on your car. There are all sorts of numbers on there that look a lot like gibberish to the untrained eye. But, armed with a little knowledge, these numbers are easily deciphered and can give you all the information you need about your tires. There are three primary areas on the sidewall you should concern yourself with, the size and service description, the Department of Transportation number, and the North American Load and Pressure Description.

The size and service description is the most important information on the sidewall. There are essentially two ways of displaying the size, Metric sizing and Flotation sizing. Metric sizing is far more common and is usually displayed in a format similar to this: 225/60R16 95H. In Metric sizing, the first number refers to the width of the tire in millimeters, in this case, 225. The second number is the aspect ratio, also known as the "series." This is the height of the tire from the bead of the tire to the top of the tread expressed as the percentage of width. In our example, the tire is 60% as tall as it is wide. The third number is the rim diameter in inches. 

The service description lists the speed rating and load index of the tire. The speed rating is the maximum speed a tire can be operated at for a length of time without failing. There are many different speed ratings, but the most common ones from slowest to fastest are: S, T, H, V, W, Y, ZZ. The load index is similar to the speed rating in that it expresses the amount of weight a tire can support in a range of values. The best way to determine the exact weight a tire can handle is to look at the Load and Pressure Description listed on the side of the tire.

Flotation sizing is much less prevalent today, and is typically on used on off-road applications. It's listed like this; 35X12.50R15. The 35 is the overall tire height from bottom to top in inches, and the 12.50 is the tire width in inches. Just like in Metric sizing, the last number refers to the rim diameter in inches.

The North American Load and Pressure Description clearly lists the maximum weight and air pressure that a tire can hold. It's listed on the side of the tire in small print in this format: Max Load 630 Kg (1385 lbs.) at Max Pressure 300 kPa (44 psi). It's important to remember that a tire can only carry the maximum load if it's inflated to its maximum pressure. The lower the air pressure, the lower the maximum carrying capacity.

The final set of numbers you need to know about is the Department of Transportation (DOT) number. This number is used by the manufacturer to determine when and where the tire was made and in which batch. It's similar to the serial number or barcode on most products you buy. This is how the tire manufacturer identifies the tire in case of a recall or other problem. The portion you need to concern yourself with is the last four digits of the DOT number. This contains the manufacture date.

The last four digits of the DOT number will be listed like this: 3401. The last two numbers, 01, are the year in which the tire was manufactured, 2001 in this case. The first two numbers, 34, are the week in 2001 that the tire was constructed in. Typically tires are guaranteed for six years after the manufacture date, but that varies between manufacturers. Tires that are too old can dry-rot or otherwise break down, resulting in failure. 

A little tire knowledge can greatly simplify your next tire buying experience. Ask questions of your tire dealer and make sure you get the proper fitment for your vehicle.

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