Sizing information molded on the tire sidewall provides a significant amount of detail about the tire. Once you know how to decode the data, you’ll be able to better understand the tire's intended purpose, dimensions, load capacity, speed and much more.
Service Type or Size Designation
Some tire size designations start with a letter(s) that identify the type of vehicle and/or type of service for which they were designed. The typical letter for passenger and light truck tires would be P, LT, T and C.
Passenger tires that are designed based on the Tire and Rim Association’s (TRA) standard start with the letter P and are known as P-metric size tires.
Passenger tires that are designed based on the European Tyre and Rim Technical Organization’s (ETRTO) standard start without any letters. These sizes are known as hard-metric or Euro-metric size tires. Euro-metric sizes are dimensionally equivalent to P-metric sizes, but typically differ subtly in load-carrying capabilities.
Light truck tires that are designed based on the TRA’s standard often start with two letters: LT. LT signifies the tire is a light truck-metric size that was designed to be used on pickup trucks, which have higher cargo carrying and towing capabilities than passenger cars.
An example of this type of sizing convention is an LT215/75R15.
In older size conventions, the LT designation may be at the end of tire size. Those sizes are known as flotation light truck size. An example of this type of sizing convention is an 35x12.50R17LT.
A tire size that begins with a letter T signifies the tire is a temporary spare or mini-spare. This kind of tire is designed to be used temporarily, only until a flat tire can be repaired or replaced.
A Euro-metric size designed for carrying heavy cargo and/or towing trailers often ends with the letter C. C signified the tire is a commercial tread, intended for vans or delivery trucks that are capable of carrying heavy loads. 195/70R15C is an example of this type of sizing convention.
The three-digit number identifies the tire’s section width in millimeters. The actual tire may not match perfectly to this, but it is close.
The two-digit number following the section width identifies the tire’s aspect ratio. Aspect ratio is simply identified as a percentage of the section height divided by the section width. In the example below, the aspect ratio of the tire is 70 percent, so the section height of tire will be 129.5mm. The higher the aspect ratio, the taller the tire sidewall.
A letter R following the tire aspect ratio identifies the tire's construction as a radial tire. In rare cases, a D will identify a bias ply construction, where the body plies crisscross on a diagonal.
In recent years, ETRTO has adopted F branding standards that allowed tire manufacturers to identify tires with self-supporting, run-flat constructions within the tire size designation.
The last two digits on tire size refer to the rim diameter, or the size of the wheel that the tire is intended to fit.
The two- or three-digits after the rim diameter, along with the letter next to it, are known as service descriptions. Service descriptions show the load index, which defines the maximum loading capacity of a tire, along with the speed rating, which determines the maximum speed allowed.
This table shows the typical passenger tire load index along with the maximum loading capacity. The maximum loading capacity at maximum pressure is also available on the tire sidewall. In this example, a load index of 116 on a P265/70R17 116 will be a maximum of 2,756 pounds of load per tire.
Passenger tires are also identified as light load (LL), standard load (SL) or extra load (XL) on the sidewall. This is the same in both P-metric and Euro-metric sizes, however, the inflation pressure at maximum load slightly varies between the two.
Light truck tires are identified by load range, which vary from load range B to load range F. The majority of tires in the market are load range C, D and E. This table shows the load index in the tire. Each load index in light truck sizes is also identified by two numbers. The first number is the maximum load in single wheel, and the second number is the maximum load in dual-vehicle applications. In this example, 121/118 represents the maximum load of 3,197 pounds in single and 2,910 pounds on a dual application.
This table illustrates the speed capability of tires. Speed ratings were established to match the speed capability of tires with the top speed of the vehicles to which they are applied. Speed rating does not suggest the speed of the vehicle, and only shows the maximum capability of the tire.
A tire’s maximum load is the most weight the tire is designed to carry. Since a tire’s load carrying capacity is related to the tire’s size and how much inflation pressure is actually used, maximum loads are rated with the tire inflated to an industry-assigned inflation pressure.
Inflation PressureA tire can only do its job of carrying a vehicle load when it’s probably inflated. Vehicle manufacturers assign a tire size with a specific tire pressure to carry the load of the vehicle and passengers, and provide the best balance of handling, traction, fuel efficiency and durability. This inflation pressure is almost always different from what is indicated on the sidewall. The sidewall indicates the maximum pressure and corresponding load for the tire, but the recommended pressure can always be found in the placard in the driver’s door panel.
In case of a tire size change, the proper inflation pressure will be recommended to maintain the same amount of load. Higher inflation pressure provides higher load capacity, which is why light load passenger tires have maximum load pressure of 35/36 psi, while extra load passenger tires have a maximum load pressure of 41/42 psi.