Base Tire Information
A tire’s base information is defined on the sidewall in a standardized format (as illustrated below). The first character defines the tire as a passenger or light truck tire as follows:
- “P” or no Letter = Passenger car tire – The tire used in this lesson does not have a beginning letter, so it is a passenger car tire.
- “LT” = Light truck tire – These tires are built heavier and are not specifically designed for use on a passenger car.
The tire’s tread section (as illustrated below) measures 225 millimeters wide.
The sidewall’s height (as illustrated below) is 50% of the tread’s width of 225 mm. Cars with small sidewall ratios are commonly called low profile tires.
YouTube – Does Aspect Ratio Matter?
The tire is a radial design meaning the cord plies of the tire are wrapped in a radial fashion other than on an angle which is called a bias-ply tire. Since the automotive industry switched to radial-ply tires in the 1970s, the “R” is not always listed, but implied.
The diameter of the rim (as illustrated below) is 17″. Note that a few tire sizes define the diameter in millimeters.
The tire has a load index of 98 which is 1,650 pounds. This is the highest load the tire is capable of carrying.
Load Index Table
The tire has an “H” speed rating which is 130 MPH. This is the highest speed the tire is designed to reach and maintain.
Speed Rating Table
Most tires have tread that works equally well in either direction, but some tires are designed to give superior performance when rolled in a specific direction. The tread on directional tires tend to look like a “V” and is designed for moving water to the outside of the tire better than a standard tread.
Directional tires will designate the direction of rotation on the sidewall as show in the image above. If a directional tire is rotated to the opposite side of a car, the tire would have to be flipped around on the rim to keep the direction of rotation correct
On an asymmetrical tire, the tread on the two sides is not the same. Often one side is designed for better cornering and one side is designed for better braking. Asymmetrical tires will designate which side of the tire goes to the outside on the sidewall.
Asymmetrical and Directional Tire
A tire that is both directional and asymmetrical is rare and is designed for high-performance applications. Since the tire is designed to roll in one direction and the inner part of the tread is different than the outer part of the tread, the tire is designed and sold as left and right side tires (two different part numbers).
Tire Build Date
Every tire sold in the US must have a Department of Transportation (DOT) serial number. The last four numbers of the serial number contain the build date code as (Week of Manufacturer | Year of Manufacture). This tire was manufactured in the 51st week of 2007,
Uniform Tire Quality Grading (UTQG)
In 1978 the National Highway Traffic Safety Administration (NHTSA) developed the UTQG system, a way for consumers to compare key features between different tires. In 1979 the Department of Transportation (DOT) federally mandated UTQG ratings on the sidewall as part of the DOT approval process,
Treadwear is an index that indicates a tire’s relative wear rate. The higher the treadwear number, the longer it should take for the tread to wear down. Each tire manufacturer builds its own control tire and assigns it a treadwear of 100. All the tires made by each manufacturer are compared to their own internal control tire. This means you can use the treadwear index to compare relative tire wear within the same tire manufacturer, but not across different manufacturers.
For example, a tire grade of 200 should wear twice as long as the manufacturer’s control tire.
Tread wear percentages of all tires currently being sold:
- 15% are rated below 200
- 25% are rated 201 – 300
- 32% are rated 301 – 400
- 20% are rated 401 – 500
- 6% are rated 501 – 600
- 2% are rated above 600
Traction grades are an indication of a tire’s ability to stop on wet pavement. A higher graded tire should allow a car to stop on wet roads in a shorter distance than a tire with a lower grade. Traction is graded from highest to lowest as “AA”, “A”, “B”, and “C”.
Traction grade percentages of all tires currently being sold:
- 15% are rated “AA”
- 77% are rated “A”
- 7% are rated “B”
- Only 4 lines of tires are rated “C”
Temperature grades are an indication of a tire’s resistance to heat buildup. Sustained high temperatures (for example, driving long distances in hot weather or carrying heavy loads at high speeds), can cause a tire to deteriorate, leading to blowouts and tread separation. From highest to lowest, a tire’s resistance to heat is graded as “A”, “B”, or “C”.
- 62% are rated “A”
- 34% are rated “B”
- 4% are rated “C”