Over time, even when just stored on a rack, tires start to degrade. Given this, it’s important to know how fresh the tires are you’re buying. The subject of “expiration dates” is open to debate since different tires age at different rates based on their construction and how they are stored. We wouldn’t buy tires that have been sitting around for over four years. One or two years hanging out in a warehouse is preferable. Every tire should be stamped with a Tire Identification Number. Much of it is unimportant and refers to where the tire was made, size, and other data. The crucial part is the last four numbers (or three numbers if manufactured prior to 2000). The first two numbers indicate the week of manufacture and the last two are the year. So the above tire was churned out in the 17th week of 2009.