Every vehicle owner will need to replace their tires at some point, so it's important to know which tire is right for your vehicle before that time comes. The information on a tire's sidewall, represented by letters and numbers, provides all the necessary details, including size, age, and safety. Understanding what these letters and numbers mean can be overwhelming, but it can be made simple with a little explanation.
A typical tire will have the following information:
P225/70 R16 91H
P - Tire Type or Service Description
225 - Tire Width
/70 - Aspect Ratio
R - Construction
16 - Wheel Diameter
91 - Load Index
H - Speed Rating
The first letter in the tire information represents the tire type or service description. It shows what the tire is intended for, or the type of service it will provide. The “P” in the tire information above means it's intended for passenger vehicles and is known as a P-Metric tire.
Other tires will have different letters at the beginning. For example, “LT” stands for “Light Truck,” “ST” stands for “Special Trailer,” and “T” stands for “Temporary.” If a tire is built for light trucks, its information will begin with the letters “LT.” A temporary tire, such as a spare, will also have the letter “T” at the beginning.
Tire information may not start with a letter for some tires, these are referred to as “Euro Metric” tires and are made to European standards. They may have different load capacities.
The first set of numbers that follows the tire description indicates its width, which is measured from sidewall to sidewall in millimeters. The number “225” in the tire information above means its width is 225 millimeters. The larger the number, the wider the tire will be.
The aspect ratio represents the height of a tire's sidewall compared to its width and is also known as the profile height, sidewall height, or section height. It is represented by the two digits after the tire width. The number “70” in the tire information above means the profile height is 70% of the tire's width.
The bigger the number, the higher the sidewalls will be. Tires with higher sidewall heights are called high-profile tires, while those with lower sidewall heights are called low-profile tires. Low-profile tires are performance tires and offer better handling and improved steering response.
The letter “R” in the tire information represents “Radial Construction” and refers to how the tire was constructed. Radial construction means the layers or internal ply cords of the tire run radially across the tire. Radial tires are the most commonly used tires.
The wheel diameter, also known as the rim diameter, is represented by a number after the tire construction. It shows the tire size from one end to the other and indicates the wheel size. The number “16” in the tire information above means it’s for a wheel with a “16” diameter.
The Load Index is an indicator of the maximum load a tire can support when inflated to the recommended pressure. It is typically represented by a number that follows the wheel size in the tire information. In the example "91", the number refers to a load index of 91.
The Load Index does not directly indicate a tire's load-carrying capacity, but instead corresponds to a specific capacity listed on a tire load index chart. Generally, the higher the number, the higher a tire's load-carrying capacity. In this example, the number "91" corresponds to a load-carrying capacity of 1,323 pounds.
There are two types of load-carrying capacities: Standard Load and Extra Load. Tires with Standard Load capacities do not have a label attached to the Load Index, as seen in the example. Tires with Extra Load capacities, however, have the letter "XL" attached to the Load Index, indicating a higher load-carrying capacity than standard load tires.
It is important to note that P-Metric and Euro-Metric tires have different load-carrying capacities. A Euro-Metric tire may have one or two points higher than a P-Metric tire, meaning a P-Metric tire cannot replace a corresponding Euro-Metric tire without reducing the load-carrying capacity. However, a Euro-Metric tire can replace a corresponding P-Metric tire, increasing the load-carrying capacity.
The last letter in the tire information sequence indicates the speed rating, which shows the maximum speed the tire is designed to handle. This rating corresponds to a list of speeds on a tire speed rating chart. The speed rating is typically selected to match the vehicle's top speed.
In the example "H", the letter indicates that the tire can safely handle speeds up to 130 mph. When replacing a tire, it is important to choose one with an equivalent or higher speed rating and to make sure all tires on a vehicle have the same speed rating.
Knowing how to read tire information is crucial for safety, as tires are the only point of contact between a vehicle and the road surface. Proper tire function is necessary for acceleration, steering, and braking. The information on a tire's sidewall includes details about its purpose, size, age, and load and speed capabilities.
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