The numbers on a tire, such as 100/90, indicate the tire's size and specifications. Here's what each number means:
The first number, "100" in this case, is the tire's width in millimeters. It refers to the distance between the tire's sidewalls when properly mounted and inflated on a rim of a specified width.
The second number, "90" in this case, is the tire's aspect ratio. It's the ratio of the tire's height to its width, expressed as a percentage.
The letter after the aspect ratio indicates the tire's construction. "R" stands for radial construction, which is the most common type of tire construction today.
The number after the letter indicates the wheel diameter in inches that the tire is designed to fit.
So, "100/90" on a tire means that the tire's width is 100 millimeters, its aspect ratio is 90%, it has radial construction, and it is designed to fit on a wheel with a diameter of 18 inches (assuming the tire is designed for a motorcycle, as this is a common tire specification for motorcycles).