What would you say if you had to describe your tires in a few words? You might say something like, “They’re black, grooved, rubbery, and round.” While you’re certainly right, you’re missing some key information—and it’s printed right on the tire! Find out what your tire numbers mean so you can make the best, most informed decisions about your tire and car care.
For the purpose of this piece, let’s pretend we’re looking at a tire with 215/65 R15 printed on its sidewall. What do the numbers on this tire mean?
Tire Width: 215/65 R15
“215” in this sequence refers to the tire’s width in millimeters. In other words, this tire is 215 millimeters wide, from sidewall to sidewall.
Aspect Ratio: 215/65 R15
“65” in the sequence is the aspect ratio of the tire. The aspect ratio is a percentage, and it tells you the ratio of the tire’s height to its width. In this example, the aspect ratio number “65” means that the tire’s height is 65 percent of its width.
Tire Construction: 215/65 R15
The “R” in this sequence refers to the construction of this tire. Specifically, the “R” means that the layers run radially across the tire. The majority of tires on the road today are radial tires, in which the internal layers are perpendicular to the axis of rotation (and the direction of travel).
The other letter you might see here is a “D,” which denotes a tire that has a diagonal or bias ply construction. These tires have plies that are laid out in a diagonal, or crosshatch, pattern.
Tire Diameter or Size: 215/65 R15
The very last number in the sequence, “15,” is the diameter of the tire. This is an important number—this is the number you’ll refer to when you need to know your tire size! The “15” indicates that this particular tire would fit a rim with a 15-inch diameter.
Now that you know how to read numbers on tires, you'll be able to describe your tires with much more knowledge and accuracy than, “black, grooved, rubbery, and round.” Check out our latest tire deals and head to your nearest Firestone Complete Auto Care to chat it up with people who speak tire, too! Our technicians are here for everything you need, from tire repair to tread depth checks.