Decoding your car's tire information doesn't require flipping through the owner's manual. All the information you need is right there on the sidewall. You just have to know how to crack the code. Not only do the jumble of letters and numbers tell you the size of the tire, but they can also tell you how safe your tires are and how old they are, too.
Let's take a look at the tires on Roadshow's long-term 2018 Kia Stinger for example.
The biggest stamping on the tire is the brand and model, in this case Michelin Pilot Sport 4. On this particular tire, there is also a note of "outside." This indicates that the tires are asymmetric and have a change of tread pattern across the tire that can disperse water while also maintaining dry grip.
You might be confused by the following string of letters and numbers: "255/35 ZR19 (96Y)." That looks like someone's email password, doesn't it? 255/35 indicates that the tire is 255 millimeters wide, with a 35-percent sidewall aspect ratio. In other words, the height of the sidewall is 35 percent of the tread width.
Yes, tires make you do math.
Multiply 255 by 0.35 to get an 89.25 millimeter sidewall height. If you want to convert either of those numbers to inches, divide by 25.4, the number of millimeters in an inch. These tires are 10 inches wide and have a sidewall height of 3.5 inches.
The "Z" indicates a speed rating of over 149 mph. There's more to it, however, and I'll explain the rest a bit further on.
"R" tells us that the tire has a radial construction. Radials are by far the most common type of passenger-car or truck tire, where the plies are situated perpendicular to the direction of travel.
In rare cases, perhaps if you drive a classic car on vintage-spec tires, you may see a "D" here, which indicates a diagonal or bias-ply construction. In these tires, the plies overlap each other on a diagonal.
You may also see the designation "F" after the construction-type symbol. This means the tire is a run flat and can be driven at slower speeds for a short distance after a puncture -- even after a complete loss of air pressure. Typically, the guideline in such scenarios is 50 mph for 50 miles.
"19" simply means the tire should be mounted on a wheel with a rim diameter of 19 inches. Luckily, no math is needed here.
On some tires you may see the note "M+S," which means the tire has been rated for mud and snow. You may also see a three-peak mountain graphic with a snowflake, indicating the tire is a step above an M+S tire for foul-weather performance.
It's important to note that neither of these symbols means the tire is specifically designed for mud or snow, just that it's been engineered to handle greater amounts of the slippery stuff than tires that don't have these markings. If you plan on driving in the deep snow or mud, you should consider a more specialized winter or off-road tire, respectively.
Locked and loaded
After the tire's size, you'll see its service description, here indicated by "(96Y)". The number is the load index. The higher the number, the more weight the tires can handle. In this case, "96" means the tire can handle 1,565 pounds, while the "Y" indicates the tire is rated to be safe at speeds up to 186 miles per hour.
But what about that "Z" rating mentioned earlier? "Z" means that the tire has been tested over 149 mph, but doesn't indicate how far over. The "Y" rating basically puts the cap at 186 mph. Except here, the designation is in parentheses, indicating that the tire has been tested above 186 mph. However, again it doesn't indicate how far over 186 mph that testing was done.
These speed ratings might matter to folks who take their Lamborghini Huracán to the track and have the capability of going over 186 mph, but for most of the driving public, a Y-rated tire, parentheses or no, should be more than sufficient for all street use.
If you'd like to see what the ratings are for your particular tire, Tire Rack has a few excellent charts here.
The Department of Transportation leaves its mark on tires as well. On the sidewall, you'll find DOT followed by a series of numbers that indicate such things as manufacturing location and specs. However, the most important numbers are the last four, indicating the week and year the tire was made. In this case "1417" means it was made between April 3-9, 2017.
This information can be useful when assessing whether tires need to be replaced (which can be particularly helpful when shopping used cars). If the tires are over five years old, regardless of tread depth, it's time to get them inspected, and most likely replaced.
This born-on dating can also be helpful when attempting to figure out if your tires are affected by a recall.
Wear and tear
The Uniform Tire Quality Grade Standards are also molded on the tires, indicating treadwear, traction and temperature tolerances. While the tests to determine these criteria rely on standards set by both the DOT and National Highway Traffic Safety Administration, the testing itself is performed by the manufacturer, or an outside company it hires.
Treadwear testing is done on a controlled test track over 7,200 miles, and the resulting rating is comparative in relation to the test tire. A treadwear rating of 100 means the test subject is estimated to last as long as the test tire. Ergo, a tire rated at 200 promises to last twice as long as the test tire, but wear out twice as quickly as a tire rated at 400.
Frankly, UTQGS isn't always the best way to determine probable wear, as the test tire runs through so few miles and wear will vary based on driving style and real-world conditions. Tire manufacturers' treadlife warranties are often a more useful indicator of projected life.
Traction rating designates a tire's wet stopping ability. The Michelins get the highest rating of AA, but you might see A, B or C here.
The temperature rating grades the tire on its ability to resist heat. A tire that can't dissipate heat will not be able to run safely at high speeds. The law requires a minimum grade of C, but the Michelins fitted on this particular Stinger go to the top of the class with an A grade.
To the max
Maximum load and tire pressure information must be marked on every tire sold in the United States. On these tires, we see noted "Max Load 710 kg" (1565 lbs) and "Max Press 340 kPa" (50 psi). This is not the same thing as the recommended tire pressure. For that information you should refer to the owner's manual or the information sticker on the driver's side door jamb.
You may also see an "E" with a number in a circle followed by some numbers. This means the tires meet European specifications, which vary slightly from US standards. Other codes include NOM for Mexico, CCC for China, SNI for Indonesia and N superimposed over an I for Brazil.
There is a lot to learn from the seemingly random assortment of numbers and letters on your tire's sidewall. Now that you know how to crack the code, you'll be able to take better care of your tires, or figure out when it's time to invest in new ones.
Originally published July 2018.