Tires are just about the most important part of your car. If they're in bad shape, the car's ability to accelerate, stop, and turn in all conditions is greatly compromised. Everybody knows to replace tires when the tread gets down to the wear bar, but what about when they get too old?
The rubber in tires deteriorates over time due to UV and environmental exposure. The resulting "dry rot" leaves tire structure brittle and leads to sidewall damage and eventual failure. This isn't "Oh I'll just fill it up and drive on it." No, this is complete loss of function.
As a precaution, the Department of Transportation recommends changing even new-looking tires once they hit 10 years old, and many manufacturers recommend swapping them out at six years old. Of course, this aging is highly dependent on where the tires live. Tires in hot dry climates have much shorter lives than those in moderate, moist climates. Still, you want to maximize tire life when buying a new set of rubber.
Since tires can sit in a warehouse for years, you'll want to know when the tires were made, rather than purchased, and there's an easy way to tell. Every tire has a "born-on date" molded into the sidewall and coded according to federal requirements. Near the edge of the rim, look for a long serial number starting with the letters "DOT." The code will end in either three numerals (pre-2000) or four (post 2000) and correspond to the two-digit week followed by the one- or two-digit year. For instance, the tire seen above has a code of DOT DA08 JM1R 3011, which would indicate a tire manufactured in the 30th week of 2011. DOT R5HG FHR 404 would indicate a tire manufactured in the 40th week of 1994 (or 1984, or '74). If you find your "new" tires are more than two years old, feel free to request a newer manufacture date or a discount from the salesman. The customer is always right, after all.
In case you were curious about other elements of the code, "DOT" means the tire has passed the Department of Transportation's testing requirements. The next two digits are the tire plant code, followed by a two-digit tire size code, and then a four-digit manufacture's code. Now your neighbors will look at you funny when they catch you staring at your tires.
