Tires (Continued)

Last article talked about different types of tires and the letters associated with each type. This article focuses on the numbers on the sidewalls of tires. Each set of numbers means something specific about a tire.

Take P225/50R16 91S for example, the “P” means passenger tire, as mentioned in the last article. The “225” describes the section width of a tire. Section width basically tells how wide the tire is, and measure in millimeters, not inches. Section width is determined by the distance between the widest part of the outside sidewall across to the widest part of the inner sidewall when mounted on the correct wheel width. 225mm can be converted into inches by dividing 225 by 25.4 ( the number of millimeter in one inch.) The result is 8.86 inches wide.

The “50” in P225/50R16 91S describes the tire’s sidewall height, from edge of the rim the top of the tread. The sidewall’s height is referred as the tire’s aspect ratio, profile or series. The “50” is a percentage of the section width, which means the sidewall height or aspect ratio is 50% of 225mm. Like section width, the section profile is measured by millimeters. To convert section profile to inches, you divide 225 by 25.4, which will tell the section width in inches. Then take 50% of the section width in inches to get the end result of 4.43” tall. Now take P255/70R16 for example, you would first convert the section width to inches, which is 8.86”. Then take 70% of the result to end up with 6.20” for the sidewall profile.


The “R” in P225/50R16 tells you about the tire’s internal construction. The tire’s internal construction is designed three ways. The “R” stands for radial construction meaning the tire’s body plies run parallel to each other around the tire. Body plies are referred to as a rubber-coated layer of fabric containing cords that run parallel to each other, extends from bead to bead and goes between the inner liner and belts or tread. The number of plies determines how strong the tire is. Radial tires are the most popular tire being sold today. They are used for cars, SUV’s and light pickup trucks.

If there’s a “D” in the tire size, like 225/50D16, it means that the internal body plies are crisscross diagonally and has a “bias ply” construction. These types tires are stronger and able to carry a heavier load. Bias ply construction tires can be used for light trucks and spare tires.

In 225/50B16, the “B” means that the internal body plies are not only crisscross diagonally, but that have reinforced with belt under the tread area. This type of construction is called “Belted.” Belted construction tires are practically extinct.

Bias ply and belted construction are not designed for speed where as the radial tires are. For more information on the internal construction of tires, go to http://www.4crawler.com/Diesel/Tires.shtml.

The “16” in P225/50R16 refers to the diameter of the wheel (or the rim) and tire. Tire and wheel diameters are expressed in inches. There are different wheel and tire sizes, anywhere from 8” to 28” that fit on all types of vehicles, trailers and trucks.

Some wheels and tires will have diameters in halves, like R16.5, R17.5 R19.5. These tires are used on some heavy-duty trailers and light trucks and cargo vans.

When a tire has a 91S after the tire size, like P225/50R16 91S. 91S stands for the tire’s service description. Service description refers to the tire's Load Index and Speed Rating. Service Descriptions are required on all speed rated (except for Z-speed rated) tires manufactured since 1991. For more information on Service Descriptions, go to http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=35.

The tire’s DOT is a 12-digit code that is on every single tire given by the Department of Transportation. It’s way to locate a tire like a tracking number. DOT provides information like: 1st and 2nd characters are the manufacturer and plant code. 3rd and 4th characters are the tire size. Characters 5th-8th identifies the brand as well as other characteristics. 9th-12th characters: The last four numbers represent the date the tire was made. The first two numbers indicate the week and the last two numbers is the year. For example 0507, indicates the tire was made in the 5th week of 2007)