Tire Size Calculator

Tire Code

Car tires are described by an alphanumeric code, as shown below. Being able to read the tire code can be important, particularly when considering getting new wheels or tires.

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.

• P stands for passenger car — intended for cars, SUVs, crossovers, minivans, and smaller pickup trucks.
• LT stands for light truck — designed to carry heavy loads and capable of towing trailers.
• ST stands for special trailer — used for boat trailers, utility trailers, and travel trailers.
• T stands for temporary — usually spare tires not intended for everyday use.

Nominal section width: The next part is a 3-digit number that describes the nominal section width of the tire in millimeters (mm). This is the widest point of the tire from sidewall to sidewall.

Aspect ratio: Following the forward slash (/), a 2- or 3-digit number indicates the ratio of the tire's section height to its width. For example, 65 means the section height is 65% of the section width.

Internal construction: A single letter (R, D, B) indicates the internal construction. R stands for radial (most common), D for diagonal, and B for bias-belted.

Wheel diameter: A 1- or 2-digit number that indicates the diameter of the wheel in inches that the tire is designed to fit.

Load index: A 2- or 3-digit number indicating the maximum weight a tire can carry at a given inflation level. For example, a load index of 95 means the tire can carry 1,520 lb at 42 psi.

Speed rating: A letter indicating the maximum speed at which a tire can carry its rated load. For example, H rating means up to 130 mph (210 km/h).

DOT Code

The DOT code is a required marking 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, "1326" means the 13th week of 2026. Tires produced more than 10 years prior should not be used regardless of their condition. Some manufacturers recommend replacing tires every six years regardless of mileage.

How Is Tire Size Calculated?

The dimensions computed by this calculator use the following equations:

The aspect ratio and tire width are from the tire code. Their product is the section height. The tire diameter is the sum of the wheel diameter (converted to mm) and twice the section height. The circumference is then used to calculate revolutions per mile or per kilometer.

What to Consider When Changing Tire/Wheel Size

There are many reasons to change tire or wheel size, often for aesthetic reasons. When doing so, it is important to consider several factors to avoid consequences such as increased wear and tear on suspension and brake components, which can affect handling or cause mechanical issues.

A common change is to increase rim size, which typically requires thinner tires to maintain the same overall diameter. Generally, it is recommended to choose tires with a diameter within 3% of the factory recommended tires to minimize side effects.

Changing tire size affects the speedometer and odometer, since they are calibrated based on the original tire size. A larger tire results in a speedometer reading that is lower than actual speed, while a smaller tire results in a reading that is higher than actual speed. The odometer is affected similarly.

When replacing tires, it is ideal to change all four at the same time. If this is not possible, at least replace tires in pairs on the same axle. It is highly recommended that replacement tires be of the same type and brand. Differences in tires can cause loss of control, inaccurate speedometer readings, or damage to stability systems.

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