How to Decode the Numbers on Your Car's Tires

The information hidden around your car can tell you a lot about the vehicle you're driving. Some symbols, like the ones on the dashboard, are placed in full view, while others are easy to miss unless you know where to look. That's the case with the numbers on your car's tires. Once you know where to find them and how to decode them, you can use these numbers to better understand your vehicle and how to maintain it.

According to Tirebuyer, a tire's code is located on the sidewall just above the rim and beneath the brand name. It may start with a letter or series of letters, such as in the example code P235/55 R 16 90S. A P in front of the number indicates that the tire is designed for passenger vehicles. This is likely the first character printed on the tires on the car parked in your driveway. Another example is LT, which means the tires are made for light trucks when the letters appear in the beginning of the code.

The next part of the number sequence is the size. The three digits after the letter prefix tell you how wide the tire is from sidewall to sidewall. A code that starts with P235, for example, has a width of 235 millimeters or 9.25 inches.

The two digits after the slash measure something called aspect ratio. If the number is 55, it indicates that the width of the sidewall from the rim to the outer edge of the tread accounts for 55 percent of the tire width. You can plug these numbers into a simple equation to figure out the width of the sidewall in millimeters. Working with the sample tire code above, you would multiply 235 by 0.55 to get 129.25 millimeters, or 5.09 inches, as your measurement.

In 98 percent of tires, the next digit that appears is an R. This signifies the tire's radial internal structure. A small fraction of tires will have a D instead, indicating that the tire was built using a rare crisscross pattern of internal plies. The two digits after this letter tell you what size wheel your tire will fit. A tire with an 16 in this spot is designed for a 16-inch wheel.

The last digits you'll see on your tire are the load index and speed rating. A 90 here tells you the load the tire is capable of bearing before it ruptures (90 equates to a maximum load of 1323 pounds, according to load index charts). Look at the letter at the end of this number to determine how fast your tires are built to go. This part of the code also requires an index chart to translate. If the letter on your tire is S, its maximum speed is 112 mph. The chart goes up to 186 mph, which is signified with the letter Y.

Many drivers get by without ever checking these codes, but it's helpful to know what they mean the next time you're shopping for tires. If you have more questions about the tires on your car, you can learn the story behind their design here.