Understanding Tire Size Markings
Tire size markings offer a wealth of information about your tire’s specifications. Found on the tire sidewall, these consist of a series of numbers and letters, like “225/50R17”. Understanding these markings lets you make an informed decision when replacing tires, ensuring optimal vehicle performance and safety.
This is the first number, representing tire width in millimeters. For instance, a tire marked 225 has a width of 225mm.
Calculated as the section width multiplied by the aspect ratio (the second number in the sequence), the sidewall height is a percentage of the tire width. Therefore, a 165mm width and 70% aspect ratio tire has a 115.5mm high sidewall.
⮞ Sidewall = Section Width × (Aspect Ratio/100)
Found by adding the rim diameter to twice the sidewall height, the overall diameter is key to ensuring a tire fits your vehicle. For a 255/70R16 tire, the overall diameter is 30.1 inches.
⮞ Diameter = (2 × Sidewall) + Rim Diameter
Tire circumference is the tire’s diameter multiplied by pi (approximately 3.14). Hence, a 255/70R16 tire has a circumference of 94.4 inches.
⮞ Circumference =Tire Diameter × π
Revolutions Per Mile
A measure of how often your tire turns over a mile can affect your speedometer reading. It’s computed as 63,360 (total inches in a mile) divided by tire circumference. So, a 255/70R16 tire makes approximately 671 revolutions per mile.
⮞ Revolutions per Mile = 63,360 ÷ Circumference
As indicated by the letter in the sequence, the tire’s construction type can significantly influence vehicle handling. “R” signifies radial construction, the most common type for passenger cars.
Lastly, the wheel size is the diameter of the wheel that the tire is designed to fit, measured in inches. The last two digits denote this, and for a 255/70R16 tire, the wheel size is 16 inches.
Meet Caitlin McCormack, a Tire Size Expert and Blogger Passionate About Everything Related to Tires. With Years of Experience in the Tire Industry, Caitlin Has Become an Expert in Tire Sizes and Their Impact on Vehicle Performance.