Learn to Read Your Tire Specification
Every vehicle has its unique tire specifications provided by the OEM. One of the basic specs is the tire size. Tire sizes can be easily found on the sidewall of the tire. Three main parts of a passenger tire size are Tire Width, Aspect Ratio, and Wheel Diameter. Reading a full tire specification correctly can be a confusing subject because there are so many different numbers and letters involved, but we will try to explain everything here. Let’s go through the main components of the example tire specification of P 225/70/R16 91S
Tire width is the width of the tire from sidewall to sidewall. The first three-digit number in the tire size represents the tire width. For example in the tire size 225/70/R16, the number 225 is the width of the tire in millimeters.
The middle number in the tire size is the aspect ratio. It is the ratio of the height of the tire's cross-section to its width. For example in the tire size 225/70/R16, the number 70 is the aspect ratio. It also means the tire height is 70% of the tire width.
The last number in the tire size is the diameter of the wheel on which the tire is installed. For example in the tire size 225/70/R16, the number 16 is the wheel diameter in inches. This number represents that the tire is made for a specific wheel size only.
Other important specs of the tire are discussed below.
The first letter before the start of a tire size represents the tire type. The letter “P” in our example tire specification means the tire is a P-metric size intended for passenger vehicle use only.
If the tire does not have any letter at the beginning or end of the three-digit numeric portion of a tire size, this indicates the tire is “Metric” size, also known as ‘Euro-metric’ tire because this naming convention originated in Europe. These two metric types may have different load capabilities.
A few more tire types with their signature letters are explained below.
T = The letter "T" at the beginning of a tire size represents a "Temporary Spare". Such tires are designed to be used for temporary purposes only until a spare tire can be replaced with a new tire.
C = If a Euro-metric sized tire ends with a "C," it represents a "Commercial" tire for carrying heavy loads such as in vans or delivery trucks
ST = The letters "ST" at the beginning of a tire size represents a "Special Trailer Service" size that was designed to be used only with trailers.
Similarly, the letters “LT”, either at the beginning or end of the tire size indicates that the tire is made for light trucks.
Sometimes, the “LT” type tires are displayed with a different tire specification such as 35x12.50R24
In this scenario, the first number represents the tire's overall diameter, or how tall it is when inflated (but not under load). In our example tire size, it’s 35 inches.
The second number in the size represents the section width, or how wide the tire is from sidewall to sidewall. (This is what the first number in metric sizes tells you). In this case it’s 12.5 inches wide.
The letter “R” means the tire has a radial construction.
The third number represents the rim or wheel size in inches (Just as in metric sizes), in this case it’s 24 inches.
The letter after the aspect ratio indicates the type of internal construction, responsible for maintaining the stability of your tire. In the tire P 225/70/R16 91S, the letter “R” indicates that this tire has radial construction. The tire’s internal plywood is oriented in a radial direction in this type of construction.
Although radial tires are the most common tire construction these days, you might also find other tire construction types such as:
Letter “D” indicates the bias-ply(Diagonal) construction.
Letter “RF” indicates a self-supporting Run-Flat construction.
Letter “RF” indicates a run-flat tire or “ZR” indicates a tire that has a speed rating higher than V.
The load index number corresponds to a specific load capacity listed in an index. The number representing the load index ranges from 1 to 150 in the index, with carrying capacities of 99 to 7385lbs.
The figure after the wheel size in the sequence P 225/70/R16 91S is your load index number.
The final letter in the sequence is the speed rating of the tire. The speed rating letters correspond to a specific speed capacity, the same way as the load index does.
For example in the sequence P 225/70/R16 91S, the letter “S” is rated for speed up to 112 mph. Similarly, the tire with speed rating “H” is rated for speed up to 106 mph.