For the average automobile owner, the numbers, letters, and words located along the sidewall of each tire may seem a bit cryptic. This unfamiliar code holds all of the information regarding the tire, including size, along with much, much more. A standard tire size may look something like this, “P260/60R17 91V.” Understanding what information that number holds will help you in the future to shop for the best deal on a new set or single replacement tire.
Tire Service Type Ratings
The first part of most tire sizes will begin with one or two letters. This letter tells what type of vehicle or what type of service the tire is designed for.
P = P-Metric
P-Metric tires are the most common tire found on the roads, and are used for the majority of passenger vehicles, hence the “P” in the tire number. These “P” tires are used on vehicles like cars, minivans, light-duty pickup trucks, and SUVs.
Metric tires, also referred to as Euro-Metric, do not have a letter at the beginning of the tire number. This tire type is referred to as a Metric tire because the sizing originated in Europe. These tires are the same as P-Metric tires in terms of sizes and dimensions, but will often have a different load-carrying rating.
LT = Light Truck
Tire numbers that begin with “LT” are made for light truck use and application. These tires are designed to handle much heavier loads than “P” or Euro Metric tires. Found most commonly on SUVs, full-size vans, and medium-duty and heavy-duty pickups, these tires are rated to handle loads up to 1-ton, such as towing a trailer or carrying a heavy load in the truck bed.
T = Temporary Spare
Tire sizes that begin with a T are meant for temporary spare use. Also referred to as mini spares or space savers, temporary spares should only be used for short-term use until the automobiles’ regular tire can be replaced or repaired.
ST = Special Trailer
For those who have boat or utility trailers, the ST, or Special Trailer, tire is designed to handle a heavy load, but is made specifically for trailer wheel sizes. These types of tires should not be used as a regular driving tire.
C = Commercial
A “C” found in tire sizes indicated that the tire is designed for commercial use. These tires, meant for application on delivery trucks or vans that carry a heavy load, are capable of handling a heavy load over a long range. They will often have a service description rating within the tire size as well.
Section Width of Tire
The first three digits that follow the service prefix are the measurement of the cross-sectional width of the tire, in millimeters. This measurement is taken by measuring from the widest point of the inner sidewall to the widest point of the outer sidewall when the tire is mounted.
Aspect Ratio of Sidewall
Following the section width measurement, a two-digit number will indicate the aspect ratio, or tire profile measurement, of the tire. This number represents a percentage of the section width measurement to determine the height of the tire’s sidewall. A lower aspect ratio number will be a lower profile tire, while a higher number will have a much taller tire with larger sidewalls.
Tire and Wheel Diameter
Immediately preceding the aspect ratio measurement is the tire and wheel diameter size. This number will determine what size rim the tire needs to be mounted on. Common tire diameter sizes include: 8, 10, 12, 13, 14, 15, 17, 18, 19, 20, 22, 23, 24, 26, and, 28. Tires within this measurement range will be most common on passenger cars, light-duty trucks, SUVs, minivans, and other daily-use vehicles.
Unique Wheel Diameters
For special applications, such as heavy-duty trucks, sports cars, or box vans, tire diameters may come in half-inch diameters. These sizes often range from 14.5 to 19.5 inches. These unique diameter wheels should never be paired with traditional wheels that measure to the inch.
More Info Found in Tire Sizes
Found in between the aspect ratio and the tire diameter, a single letter; R, D, or B, can be found to indicate the tire’s construction. The most common construction type for wheels is radial, indicated by the letter R. Over 98% of tires on the road today are radial-built tires. If there is a D, the tire has a bias ply construction, and a B means the tire has a belted construction. Belted tires have become increasing uncommon as tire technology continues to advance.
At the very end of the tire size number, a single letter may be found to indicate the speed rating. Today, only the letter Z is used here, to indicate a Z-rating, which is for sports cars.
Service Description Rating
For the majority of cars on the road, the service description rating is how you can determine what load index and speed raging you’ll need. This number and letter combination can be found at the very end of the tire size measurement. The first number identifies the tire’s load index, and the following letter will identify the speed rating.