The complex code on a tire’s sidewall isn’t just a random manufacturing number. It’s actually the tire size and indicates if the tire will fit on your vehicle.
Each letter and number have a meaning, and we explain it briefly on our website. We’d like to go into more detail about these sizes, so car experts and beginners alike can understand they mean.
For our blog, we’ll be using the tire size P 225/55R18 97T as an example.
The first part of the size stands for the class of the tire. One of four letters will be used:
LT: Light Truck
ST: Special Trailers
If the tire is European, it will not have this letter.
The width of your tire measures from the widest point of the inner sidewall to the widest point of the outer sidewall. This measurement is in millimeters and is also referred to as the section width.
This number is also known as a tire profile measurement. It indicates the percentage of a tire’s section width, measuring the sidewall distance from the wheel rim to the outside of the tread. 55, in this example, means that the sidewall distance is 55% of the section width.
This letter is the tire’s internal construction. There are three construction types.
R: Radial, meaning the internal body plies reach outward from the center.
D: Diagonal, meaning the internal body plies cross each other.
B: Belted, meaning the internal plies have a diagonal pattern, but with reinforcing belts underneath the tread.
This measurement signifies which wheel the tire should be matched to. 18, in this example, stands for 18 inches, meaning this tire needs to be matched to an 18-inch diameter wheel.
The load index lets drivers know the maximum load the tire can support. This load index chart shows 97 can support 1,609 lbs.
Just like the load index, we need a speed rating chart to understand what this part of the size stands for. T, for example, means the tires can handle 118 mph.