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Ever wondered what the numbers on the side of your tires mean? Learn how to quickly know how to shop for new tires in the right size and fit. 

How Tires Are Measured

Tires need to be able to support the weight of the vehicle and remain in contact with the road at high speeds, but different vehicle types have varying requirements. The seven unique components on the tire sidewall reveal important information about the specifications to which your tire has been made. Among them are the load capacity, vehicle type, and width of the tire from sidewall to sidewall. Getting each of the seven components just right for your vehicle provides confidence that they will perform just as needed. 

What Tire Sizes Mean

The series of letters and numbers on the side of the tire make up a tire size. Here is a breakdown of what each number and letter in the series means:

Vehicle Type

The tire size identifier begins with letters to indicate the type of vehicle it is meant to be used for. P indicates use on a passenger vehicle, while other common notations are LT for light truck or ST for special trailer.

Tire Width

This three digit number is the distance in millimeters from sidewall to sidewall of the tire. 

Aspect Ratio

The tire width is followed by a forward slash that points to the aspect ratio of the tire. The number following the slash is a percentage that compares the height of the sidewall (from the wheel rim to the top of the tread) to the tire width.

Construction Type

Tires come in three construction types: radial, belted bias, or diagonal, represented as an R, B, or D respectively. These methods refer to the way that the strong cords of polyester, steel, fabric, and rubber are attached to one another to make up the inner part of the tire. 

Wheel Diameter

The numbers that follow construction type indicate how wide, in inches, the wheel is from the two bead inner seat areas. 

Load Index

The last set of symbols on a tire are separated from the ones mentioned previously by a space, but they are among the most important as they reveal how much of a load the tires can support when fully inflated. The two to three digit number represents the maximum weight in pounds the tires can carry and are understood using a load index table for the exact weight.

Speed Rating

The speed rating is listed as a letter that corresponds to a maximum speed at which the tires can support its load capacity. The speed rating requires a table to convert the letter into the appropriate miles per hour rating. 

 

Having a full understanding of the meaning of the numbers and letters on the side of tires can help ensure that you always have tires of the perfect fit on your vehicle.