Tire size specifications are outlined by the numbers and letters found on the sidewall of tires.
It is important that we understand tire geometry and be informed of the parameters that govern their performance. Tire sizes are defined by specifications such as the type, width, aspect ratio, speed rating, tire construction type, and service description.
There are two organizations - Tire and Rim Association (TRA) and European Tyre and Rim Technical Organisation (ETRTO) - that influence tire standards.
Example of Tire Size Specifications
The following is an example of tire specifications. We will be using it as an example throughout:
- P 185 / 60 R14 82H
Tire sizes will often start with letters. For example P, LT, T, and ST.
When we use tire type “P” or P-Metric, it means that the tire is made according to certain standards of the United States (drafted by TRA) for passenger vehicles.
Learn More: The Different Types of Cars
If there are no letters at the beginning, it refers to the Euro-metric (drafted by ETRTO). The P-metric and Euro-metric differ in load capacities.
LT or Light Truck are tires for towing trailers or have a 3/4 to 1 ton load capacity.
A tire labeled “T” or Temporary Spare is meant to be used in the event of a flat tire.
Tires labeled “ST” or Special Trailer are meant for trailers. Including but not limited to boat trailers, utility trailers, and 5th wheels; ST tires should never be used on cars.
The tire width is measured from sidewall to sidewall of the tire. It is represented by a three digit number and measured in millimeters.
In our example above, the width is 185mm.
Aspect ratio is the ratio of the height of the tire’s cross-section to its width. For example, the number 60 in the above illustration represents that the height is equal to 60% of the width.
This factor shows how sensitive the tire’s reaction to the texture of the road is; The bigger the aspect ratio, the thicker the sidewall of the tire. Heavy vehicles like buses and trucks have a larger aspect ratio, which may compromise the handling.
However, they provide a more comfortable ride, as there is more air in the tires, which can cushion things like bumps.
Construction denotes the style of the tire. The letter “R” in the above example shows the tire construction type as ‘radial.’
R or “Radial” tires are the most popular style of tire. In this setup, the carcass, the cable plies radiate around the axis of the tire. The crown consists of plies that form a belt.
Typically radial tires are going to have a better grip on the road which results in a more comfortable ride. These tires will also wear evenly since the point of contact is relatively uniform throughout.
B or “Bias” feature a carcass that consists of diagonally oriented cable plies. This is often referred to as crossply.
The footprint of this tire is longer in comparison to the radial tire, it is also skinnier, which provides less grip on the road and offers a more rigid ride. At high speeds, bias tires can also deform which can affect handling.
The size of the wheel from one end of the rim to the other is the wheel diameter. It is denoted in inches.
Learn More: The Difference Between Wheels and Rims
Since the wheel mounts the tire, when we need to use a spare tire, the wheel diameter parameter will help us decide if the spare/replacement will fit on the wheel.
The load index is the maximum load that the tire can support. The bigger the value, the more load that it can bear when properly inflated. These load indexes correspond to carrying capacities. For instance, in our example, “82” is 1047 lbs.
Note: Light truck tires have two load indexes, unlike passenger tires, because LT tires are mostly used on vehicles with dual rear wheels. The first load index marks the load capacity when used as a single tire. The second index marks the load capacity when used for two tires.
Tires typically range from 70-126 for their load index. You can review the table below to find your load index rating and the corresponding carrying capacity.
Load Index Values
|Load Index||Carrying Capacity (lbs)|
The speed index rating indicates the ability of the tire to handle a maximum speed corresponding to the maximum load, without suffering any deterioration in performance.
For example, an H rating has a maximum speed capability of 130 mph or 210 km/hr. On the other hand, a ‘Z’ rated tire has no maximum speed. This rating is for more than 149 mph; Sports cars mostly use the Z rating.
Speed Index Rating
|Speed Rating||Miles/Hour (mph)|
There could be additional parameters for 2 or 3 wheelers, buses, agricultural vehicles, etc. These indicate different purposes. For instance, M/C indicates that the tire is for motorcycles only.
P 185 / 60 R14 M/C 82H
In addition to the tire size, there are other parameters on the tire sidewall as well. A few of them are:
- The brand name
- Tire performance specification criteria
- Winter tire symbol
- UTQG ratings
- DOT tire identification number
- Maximum cold inflation load limit