255/40R18 Tire Size in Inches and Millimeters

The tire size 255/40R18 is a common size used on many trucks, SUVs, and other larger vehicles.

Here is what each part of the size designation represents:

• 255: This number represents the width of the tire in millimeters. In this case, the tire is 255 millimeters wide.
• 40: This number represents the aspect ratio of the tire, which is the height of the tire's sidewall as a percentage of its width. In this case, the sidewall height is 40% of the tire's width.
• R: This letter indicates the tire's construction type. “R” stands for radial construction, which is the most common type of tire construction today.
• 18: This number represents the diameter of the wheel that the tire is designed to fit on, in inches. In this case, the tire is designed to fit on a 18-inch wheel.

How To Convert 255/40R18 Into Inches

The 255/40R18 conversion is fairly simple. The first number in the tire size, 255, represents the width of the tire in millimeters. To convert this to inches, we divide 255 by 25.4 (since there are 25.4 millimeters in an inch). This gives us approximately 10 inches.

The second number in the tire size, 40, represents the aspect ratio or the height of the tire as a percentage of its width. To calculate the height in inches, we multiply 10 by 0.40, which gives us approximately 4 inches.

Finally, the last number in the tire size, 18, represents the diameter of the wheel rim in inches. Therefore, the tire size 255/40R18 can be expressed in inches as roughly 10 inches wide, 4 inches tall, and designed to fit a 18-inch wheel rim.