21 inch tires. R21 tire size
Below we list all tire sizes with rim diameter equaling 21”. Many people believe the letter “R” stands for the radius of the tire which is wrong. In reality, this means the radial type of the tire.
A radial tire (more properly, a radial-ply tire) is a particular design of vehicular tire. In this design, the cord plies are arranged at 90 degrees to the direction of travel, or radially (from the center of the tire). The radial tire revolutionized the world of tires when it was invented in 1946.
It you want to increase the rim diameter of your car, you will have to lower the aspect ratio of your tires to preserve the overall height of a tire. While low-profile tires are more adapted for sport style of driving, high-profile tires, on the contrary, are a better choice when driving off road. High aspect ratio facilitates a deeper protector which improves the flotation ability. On top of that a higher aspect ratio may increase the clearance of the vehicle.