How to Read Tire Sizesby Andrew Chen
Section Width<p>The first three digits indicate the section width, or cross section, of the tire in millimeters.</p>
Sidewall Aspect Ratio<p>The two numbers following the section width identify the sidewall aspect ratio, and is commonly referred to as the tire's profile or series. The aspect ratio is presented as a percentage of the section width. In 245/45R18, the 45 denotes the sidewall height is 45 percent of its section width of 245 millimeters, making it 110 millimeters tall or 4.34 inches.</p>
Using a tire with a different aspect ratio from the Original Equipment Manufacturer (OEM) specs will cause the speedometer to be inaccurate.
Internal Construction<p>The letter following the aspect ratio indicates the type of internal construction.</p> <ul> <li>R: Radial - most common construction for modern tires.</li> </ul> <ul> <li>D: Diagonal - usually found in light truck tires and spare tires.</li> </ul> <ul> <li>B: Belted - an outdated construction method that is no longer used.</li> </ul>
High performance tires with a Z-speed rating (a maximum speed capability greater than 149 MPH) will have the letter Z in front of the internal construction letter. For example, 245/45ZR18.
Wheel Diameter<p>The last numbers in the sequence indicate the wheel diameter (in inches) that matches the tire. This means that the 245/45R18 tire will mount properly onto an 18-inch wheel.</p>
Always confirm that the tire and wheel diameters match before mounting the tire onto the wheel.