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DOT Codes

As part of the DOT code, there is a tire manufacture date stamped on the sidewall. Take a look at yours - there will be a three- or four-digit code. This code denotes when the tire was manufactured, and as a rule-of-thumb, you should never use tires more than 6 years old. The rubber in tires degrades over time, irrespective of whether the tire is being used or not. When you get a tire change, if you can, see if the tire place will allow you to inspect the new tires first. It's not uncommon for these shops to have stuff in stock which is more than 6 years old. The tire might look brand new, but it will delaminate or have some other failure within weeks of being put on a vehicle.
Reading the code: The code is pretty simple. The three-digit code was used for tires manufactured before 2000. So for example 306 means it was manufactured in the 30th week of 6th year of the decade. In this case it means 1986. For tires manufactured in the 90's, the same code holds true but there is a little triangle after the DOT code. So for this example, a tire manufactured in the 17th week of 1996 would have the code 176.  After 2000, the code was switched to a 4-digit code.  Same rules apply, so for example 3506 means the tire was manufactured in the 35^th week of 2006.