How Tires Are Measured

taking a precise measurament

If you have ever had to purchase a new tire, you know how cryptic the tire size coding system seems. But, these multi-digit and lettered codes actually make sense. You just have to know how tires are measured, to understand them. Below, we offer some guidance into how these essential car parts are measured.

How a Tire Is Measured

Tire measurements are labeled on the sidewall. This sizing information, such as P225/50R16, provides more detail than you may realize. The numbering sequence actually relays the tire’s specifications, including:

  • Purpose
  • Dimensions
  • Load capacity
  • High temperature/high speed

Tire Purpose or Service Type

Tire sizing starts with one or more letters. These letters indicate the vehicle type for which the tire is designed. For example, in P225/50R16, the “P” indicates the tire is intended for passenger vehicles, such as cars, sport utility vehicles, light duty trucks and minivans. P-rated sizes are the most widely purchased tire sizes today.

If there is no letter at the beginning of the series, such as for 225/50R16 92S, the tire is metric sized. These are most often used on cars in Europe. But they are also sometimes used on SUVs and vans. The difference between these and P styles is typically that of load bearing ability.

If the tire size starts with a “T,” as in T125/90D16 98M, the T represents temporary use. This is typical for a temporary spare tire used only until replacement by a full-sized tire.

When the tire size begins with “LT,” such as with LT245/75R16 108/104S, the tire is intended for use on light trucks capable of transporting heavy cargo. It is also used for those towing large trailers. These vehicles are typically medium and heavy-duty pickup trucks, full-sized vans and SUVs.

Some tire sizes end with the letters “LT,” such as with 7.50R16LT 112/107Q. These are also intended for light trucks that carry heavy cargo or tow large trailers. They use wide base sizes to enable the vehicle to drive well on loose soil or sand. Trucks with wheels of this size typically carry loads of 1/2 to one ton and include SUVs of the same class. These tire sizes are smaller versions of 18-wheel tractor-trailer styles.

If the tire size ends with a “C,” such as with 195/70R15C 104/102R, this indicates the tire is designed for heavy load-bearing commercial vehicles like vans or delivery trucks.

A tire labeled with an “ST” at its beginning is one designed for “special trailer” service. This indicates appropriate use on a boat, utility or car trailer. These never belong on a passenger vehicle and are only sufficient for trailers.

Section Width

After the letter indicating vehicle type or service use of a tire, you can find a three-digit number series. This series indicates the tire section width. The number represents millimeters, such as the “225” in P225/50R16 91S. The associated tire is 225mm across from the outer sidewall’s widest point to the inner sidewall’s widest point.

Sidewall Aspect Ratio

After the three digits indicating section width, the tire number contains two digits that identify its aspect ratio. For example, in P225/50R16 91S, the “50” indicates the sidewall height measured from the rim to tread. This is also called the tire series or profile.

Internal Construction

In the case of P225/50R16 91S, the “R” after sidewall aspect ratio represents the tire’s internal construction. An R indicates radial construction with the body plies radiating out from the wheel’s center. More than 98 percent of all tires made today feature radial construction.

When a “D” exists after the sidewall aspect ratio, the body plies align diagonally. This is called bias ply construction, such as for a spare tire or those used on light duty trucks. When a “B” replaces the R, the tire body plies cross the tire diagonally and have belt reinforcement beneath the treads. This is a belted tire, one rarely seen on modern vehicles.

European tires sometimes contain the letter “F” after the “R” or other internal construction designation. This represents a self-supporting run-flat construction, such as in the tire size 225/45RF17 91Y.

Speed Rating

Only a Z-speed rated tire features its speed rating in its size. An example is P225/50ZR16. In this tire size, the ZR represents Z-rating for speed and internal radial construction of R. All other speed ratings besides Z feature within the tire Service Description, instead.

Tire and Wheel Diameter

In P225/50R16 91S, the “16” indicates tire and wheel diameter. When buying a tire, you match these diameters for proper fit. These either appear in inches or millimeters. Inch sizing is made obvious through diameters like 8, 10, 12, 13 and so on, up to 28. You can find these sizes on most car, SUVs, minivans, vans and light duty trucks.

Another variation on the inch sizing is half-inch sizing. These typically appear as 14.5, 15.5, 17.5 or 19.5 in the tire size sequence. A tire with half-inch sizing generally fits heavy-duty trailers, box vans and heavy-duty light trucks.

A millimeter-sized tire, such as “390” in 190/65R390, typically comes from Michelin and fit late-1970s to 1980s vehicle models.

Whichever type of sizing your vehicle uses, it is critical that the tire and rim sizes match before mounting the tire on the rim.

Service Description

In P225/50R16 91S, the “91S” indicates the tire service description. This, in turn, identifies the load index and speed rating for the tire. Any speed-rated tire made since 1991 must feature a service description in the sizing.

As you can see, tire measurements make better sense when you understand what the number and letter sequences mean. Broken down into their parts, these sequences identify a lot about the tire, its construction and its appropriate use. Knowing your tire measurements also makes shopping for the right fit much easier.

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