"Born on" dating
Among the factors that influence tire performance is how recently the tire was crafted. Just like Budweiser beer, tires bear a "born on" date. Imprinted on one or both sidewalls is an alphanumeric code of a dozen or so letters and numbers that begins with "DOT." Tires produced starting in 2000 have a four-digit birthdate code at the end of the DOT number. The first two numbers indicate the week, and the last two the year. So, for example, a DOT number ending in 0205 means the tire was produced the second week of January 2005.
This date is important because rubber deteriorates with age. A tire that's been in service for five or six years should be replaced regardless of tread depth. Cut a year or two off that for vehicles that are not garaged during the day or are run in areas of high ground-level ozone.
Is "plus sizing" a plus?
Not long ago, 17-inch-diameter wheels were limited to upper-end sports cars, while 20-inchers were strictly for lunatic-fringe tuners. Now, many minivans come standard on 17s, and 20-inch "dubs" are original equipment on pickups. If you're buying a new car, should you go for the optional 19s? If your considering an "upgrade" to your current vehicle, should you go to 20s, 24s or 26s?
Here's the short answer: If show is more important than go and the roads in your region are so smooth you think a pothole is something that causes pasta sauce to leak onto the stove, then, sure, go for the bigger size. Just be sure to keep the OE wheels for when you want to sell the car because those certain-to-no-longer-be-trendy versions will be lucky to bring 50 cents a pound at the aluminum recycler.
The long answer is that there's no performance advantage or even steering-feel improvement to be gained by increasing wheel diameter beyond about 17 or 18 inches for most cars. When asked what diameter Formula One wheels would go to if the current 13-inch mandate were removed, the head of Michelin's F1 program predicted 17 to 19 inches, adding "certainly not 20 or beyond."