The federal government requires that all tires sold in the U.S. contain a Department of Transportation number. This “DOT” number or code contains critical information about the tire. Unfortunately, the vast majority of consumers do not know what a DOT number is and what information it provides. Even when they do, it can be difficult to locate and is usually found on the sidewall facing the axle, requiring consumers to crawl underneath the vehicle to decipher it. The DOT codes generally consists of eleven to 14 characters of numbers and letters and is found just above the rim on the sidewall.

 
The most critical information contained in the DOT code is the date of manufacture of your tire. This can be found in the last 3 or 4 digits of the code and will always consist of number, not letters. To determine the age of your tire, find the last three or four numbers on the DOT code – these represent the week and year the tire was built. As an example, the last three numbers "399" might signify that the tire was manufactured on the thirty-ninth week of 1999. Prior to the year 2000, most manufacturers used a 3 digit code, which has caused some confusion. This means that a DOT code ending in “399” may have been made in the 39th week of 1999 or 1989. This issue has been resolved for all tires made after 1999 with the requirement of a four digit code; two for the week and two for the year.

 
The DOT number also tells you who manufactured your tire. The first two characters of the code stand for the manufacturing facility. There are literally hundreds of off-brand tires on the U.S. Market today. Every tire manufacturer makes off-brand tires. For example, more than four different manufacturers make a “Hercules” brand tire and the only way to know who manufactured a tire is by the plant code. As of 2008, there are over seven hundred fifty (750) new tire manufacturer plant codes.