When looking at a tire's size, it is shown within the markings on the sidewall of every tire. The combination of letters and numbers make up codes that can be used to determine a plentiful amount of information about your tire. Whether it be knowing what tire size you’re running, to the speed rating, tread life and construction, knowing how to read the sidewall codes can assist with easily describing a tire in detail.
P-METRIC SIDEWALL CODE
One of the most common American tire manufacture sidewall codes, the P-Metric sidewall code is the first we shall be looking at. If your tire is P-Metric if the sidewall code begins with the letter "P". The P-Metric system became standard after being introduced by American tire manufacturers in 1975. It uses metric measurements and meets US-based standardized organization requirements. Below is an illustration of a P-Metric sidewall code, and describes what each character means:
Tire Class
The first letter in a P-Metric code is a letter representing the tire’s class. “P” represents a P-Metric passenger constructed tire built to US standards. Other codes may include “LT”, designating it to be a light truck tire. While LT tires are not part of the P-Metric family, they are still built to US standards. If there is an absence of a letter, this indicates a passenger tire constructed to meet European standards.
Section Width
After the tire’s class, there will be a number representing the section width. This measures the tire’s width from sidewall to sidewall in millimeters. This section width will be a 3-digit number. For this example, the tire’s section width is 205mm.
Aspect Ratio
The aspect ratio will follow the section width. This refers to the sidewall height as a percentage of the section width. This will always be found after the slash and will be a two-digit number. On our tire, the aspect ratio is 65, meaning that its sidewall height is 65% of the 205mm sectional width.
Tire Construction
The tire construction is found after the aspect ratio. This letter describes the tire’s composition, and how the plies are run through the tire.
- “R” stands for radial construction. This is the most common construction for everyday vehicles you would see today. Radial construction is where the tire’s plies run at 90 degrees to the centerline of the tread. In our case, this is shown in our example above of a “R” constructed tire. Radial constructed tires give vehicles lower rolling resistance, higher mileage, and a more comfortable ride compared to other construction methods.
- “D” stands for diagonal bias construction. This is where the plies are run at angles lower than 90 degrees. It is one of the least common of the construction methods, as bias belted construction had many drawbacks. Most notably was a high rolling resistance. The more rolling resistance a vehicle would have from these tires, the more gas that vehicle would have to use.
- “B” rated tires stand for bias belted construction. In an effort to address the drawbacks of diagonal bias tires, bias belted tires were developed and incorporated into many vehicles! Bias-belted tires are still on the market today. They are manufactured similarly to the original bias tire. The difference comes after the bias plies have been layered. Above the top layer of tire ply, stabilizer belts, generally made of steel or other corded material, are applied at different angles to the plies, offering additional support and a stiffer inner material to bond with the tread. Bias-belted tires provide a smoother ride and lower rolling resistance than bias tires do. These kinds of tires are popular with people who drive classic cars and who prefer to keep the equipment as close to original as possible. Bias-belted tires are also manufactured for some light trucks such as pickup trucks and SUVs! However, bias belted tires can never match the capabilities of modern radial constructed tires.
Wheel Diameter
Finally, the wheel diameter will be the last figure in a P-Metric sidewall code. This will indicate the diameter of the wheel of which the tire can be mounted on. The example tire above has a wheel diameter of 16, meaning the tire is meant to be mounted on a 16-inch wheel.
High Floatation Tire Sidewall Code
Here we will be looking at the sequence codes for high floatation tires. These tires use a different sequence of letters and numbers on the sidewall than traditional P-Metric codes, among others. Below will be an example illustration of a high floatation code:
Tire Diameter
In high floatation codes, they start by listing the tire’s diameter. This is a measurement of the tire’s height in inches. The diameter will be a two-digit number. In our example the tire has a diameter of 35 inches.
Tire Width
Next in high floatation tire codes, the tire’s width is listed in inches. In the example, the tire has a width of 12.50 inches.
Tire Construction
Tire construction comes next in a high floatation tire code. “R” or “D” codes will be shown, representing either radial construction or diagonal bias construction. In our case, the tire shows an “R” rated construction. This means the tires plies run at 90 degrees to the centerline of the tread.
Wheel Diameter
After tire construction, the wheel diameter is shown. This indicates the diameter of the wheel that the particular tire can be mounted on. In the example tire the wheel has a diameter of 17, or in other words a 17-inch wheel.
Load Range
Floatation tire codes end by providing the load range. This shows a comparative idea of the tire’s capacity to hold air and the amount of weight it can withstand. In our example tire the load range is rated at “E”. This means that the tire has a max load pressure of 80 psi, or 550 kPa.
