How to Read a Tire Code
Car tires are described by an alphanumeric code. Being able to read the tire code can be important, particularly when considering getting new wheels or tires, as being able to read the tire code is necessary for calculating things like tire size. Each part of the code describes some characteristic of the tire.
Intended Use
The first part of the code is made up of 1 or 2 letters (P, LT, ST, T) that describe the type of vehicle that the tire is intended to be used with. There are 4 vehicle classes:
- P stands for passenger car. Tires with the P specification are intended for use with cars, SUVs, crossovers, minivans, and smaller pickup trucks.
- LT stands for light truck. Tires with the LT specification are designed to carry heavy loads and are capable of towing trailers.
- ST stands for special trailer. These types of tires are used for trailers such as boat trailers, utility trailers, and travel trailers.
- T stands for temporary. Tires with the T specification are usually spare tires that are not intended for everyday use.
Nominal Section Width
The second part of the code is a 3-digit number that describes the nominal section width of the tire in millimeters (mm). The nominal section width is the widest point of the tire or the width from sidewall to sidewall. For example, a "215" means that the nominal section width of the tire is 215 mm.
Aspect Ratio
The next part of the code follows the forward slash ( / ). It is a 2- or 3-digit number that indicates the ratio of the tire's section height to its width. The section height is the height of the tire measured from the rim to the outer edge of the tire. A tire with the code xxx/65 indicates that the section height of the tire is 65% of its width. For example, for a tire with code 200/65, the section height is 200 × 65% = 130 mm.
Internal Construction
The next part of the code is a single letter (B, D, R) that indicates the internal construction of the tire. The inside of a tire is made up of plies of cord, referred to as the tire's carcass. It is an optional letter in the tire code, and if it is omitted, it is assumed that the tire is a cross-ply tire.
- R stands for radial. Almost all passenger cars use tires with radial internal construction. The cords run along the circumference of the tire, perpendicular to the direction of travel.
- D stands for diagonal. The cords run along the circumference of the tire at a 30-40° angle from bead to bead, and the layers crisscross over each other.
- B indicates a bias-belted tire. Similar to diagonal tires, except it has an additional layer above the topmost layer of ply comprised of stabilizer belts (typically steel) for added stiffness.
- Cross-ply tires are tires whose internal construction is made of nylon cord placed diagonally across each other, usually at an angle of 55°.
Diameter of the Wheel
The next part of the code is a 1- or 2-digit number that indicates the diameter of the wheel, in units of inches, that the tire is designed to fit.
Load Index
The next part of the code is a 2- or 3-digit number referred to as the load index of the tire. The load index indicates the maximum weight that a tire can carry at a given inflation level. For example, a load index of 95 means that a tire can carry 1,520 lb (690 kg) at a tire pressure of 42 psi.
Speed Rating
The last part of the code is a 1- or 2-digit/letter that indicates the speed rating of the tire. The speed rating is the maximum speed at which a tire can carry a load that corresponds to its load index. For example, a tire with a load index of 95 and speed rating of H can carry 1,520 lb (690 kg) at a tire pressure of 42 psi and a speed of 130 mph (210 km/h).
DOT Code
The DOT code is a required code in the United States that specifies the company, factory, batch, mold, and date of production of the tire. The date of production is specified with 2 digits for the week of the year, and 2 digits for the year. For example, the code "1324" means that the tire was produced on the 13th week of the year 2024. Due to the aging of the rubber, tires that were produced more than 10 years prior should not be used. Some manufacturers recommend replacing tires every six years regardless of the miles driven.
How is Tire Size Calculated?
The dimensions computed by our calculator use the following equations:
- Section Height = Aspect Ratio × Tire Width
- Tire Diameter = Wheel Diameter + (2 × Section Height)
The aspect ratio and tire width are measurements acquired from the tire code. The tire width is the 3-digit number preceding the forward slash ( / ) in units of millimeters. The aspect ratio is the 2- or 3-digit number immediately following the forward slash. Their product is the section height.
Once the section height is computed, the tire diameter can be calculated. The wheel diameter is the 1- or 2-digit number after the letter that indicates internal construction (specified in units of inches). The tire diameter is the sum of the wheel diameter and twice the section height.
What to Consider When Changing Tire or Wheel Size?
There are many reasons that a person may change their tire/wheel size. Often, it is done for aesthetic reasons. When changing the size of a tire or wheel, it is important to take certain things into consideration. Otherwise, there can be significant consequences, such as increased wear and tear to the vehicle's suspension and brake components (particularly anti-lock brakes), which can affect handling or cause other mechanical issues affecting the transmission of the car.
A common change is to increase the size of the rims. Generally, larger rims must be used with thinner tires because the tires must be small enough to fit inside the wheel well of the car. It is recommended to go with tires that have a diameter within a 3% difference from the factory recommended tires.
When replacing tires due to blowouts or wear and tear, it is ideal to change all tires at the same time. If this is not possible, at least the front two or back two tires should be changed together. Differences in tires can cause issues including loss of control, inaccurate speedometer readings, or damage to the vehicle's stability system calibrations.
Changing tires also affects the speedometers and odometers of a car, since they are calibrated based on stock tires. Larger rims and tires increase the total circumference of the wheel. This results in inaccurate speedometer and odometer readings because both are based on how many revolutions the tire makes. A larger tire will result in a speedometer reading that is lower than your actual speed; similarly, an odometer reading that is lower than your actual distance traveled.