In a tire size, the "R" stands for "radial." The letter "R" indicates that the tire has radial construction, which is the most common type of tire construction used in modern vehicles.

Radial tires have layers of cords or plies that run radially across the tire from bead to bead. These cords are typically made of steel or synthetic materials like polyester, aramid, or nylon. The cords are layered at a 90-degree angle to the direction of travel, forming a strong and flexible tire structure.

It's important to note that most modern tires are radial, and the "R" designation has become the standard for passenger vehicles. If you see the letter "R" in your tire size, it simply confirms that your tires are of radial construction.