You can maintain your tires in optimal condition and increase your fuel efficiency by understanding what the numbers on them imply. When a mechanic or tire expert asks you what kind of tires you are looking for, knowing tire sizes also helps you avoid awkward situations.

Decoding the Numbers on Your Tires

Your tires' type, width, aspect ratio, construction, diameter, load index, and speed rating are all indicated by the letters and numbers that read from left to right. Since the arrangement is industry-standard, tire numbers have the same meaning across manufacturers. These numbers are also listed on the label that is located inside your driver-side door.

 

 

 

Type

The first letter on the left is the tire type.

• P: Passenger. This is the most common tire type. It’s used on most cars, SUVs, minivans, and smaller trucks for everyday driving.
• LT: Light Truck. Light truck tires are more sturdy than passenger tires. Their heavy-duty design for SUVs and trucks can handle large loads and more rugged terrains.
• ST: Special Trailer. These tires are specifically for towed items like trailers and campers. They should never be used on a drive or steering axle.
• T: Temporary. A temporary tire is your spare. It’s only a short-term fix for a flat tire and should be replaced with a permanent tire as soon as possible.

Width

The width of your tire, expressed in millimeters from sidewall to sidewall, is indicated by the next three figures. To distinguish this three-digit number from the aspect ratio, a forward slash is always used after it.

Aspect Ratio

Interpreting the aspect ratio is a little trickier. This figure is a percentage that shows the relationship between the tire's sidewall height and width.
The sidewall's ratio to tire width is computed by measuring it from the wheel rim to the top of the tread. A 50 following the forward slash, for example, would indicate that the tire height is equal to half of the tire width (50%). The sidewall increases with the number.

Construction

The next letter tells you about the tire’s construction.

• R: Radial Tires. Most vehicles have radial tires. These are designed with side-to-side, overlapping layers (or plies) of steel and strong fabric cords coated in rubber.
• D: Diagonal Tires. Some tires have diagonal plies. They may also be called bias-construction, conventional, x-ply, or cross-ply tires. The internal layers of these tires crisscross each other at an angle.
• B: Belted Tires. Occasionally you might see a vehicle with belted tires. These are diagonal tires reinforced with a belt made of steel or another strong material.

 

 

 

 

Diameter

The next two numbers tell you the wheel diameter, from rim to rim, in inches. The inside diameter of the tire needs to match the wheel precisely to mount correctly.

Load Index

The tire load index may have two or three digits. It indicates how much weight the tire can support when properly installed and inflated. The higher the number, the more weight the tire can carry. The index number is different from the number of pounds the tire can support. You can use a load index chart to find the poundage associated with the index number on your tire.

Speed Rating

The last letter identifies the top speed at which the tire is rated. We don’t recommend testing the limits for accuracy, but it’s good to know anyway. Again, you can use a chart to determine exactly what each letter means.

 

 

 

Other Numbers and Information

You may also spot other information close to the rim of your tire.

One notation starts with DOT. This indicates the tire meets national safety standards and identifies where and when it was manufactured.
The other information is the UTQG (Uniform Tire Quality Grading), which tells you how the tire compares to others regarding treadwear, traction, and temperature.