Understanding Tire Sizes

You can find a significant amount of information about a tire by looking at the size markings that are branded onto its sidewall. Specifically, you can discern the tire's dimensions, load capacity, temperature and speed durability, and its intended purpose.

Service Type Rating

Most tire sizes begin with one or more letters that indicate the type of vehicle or type of service for which they are designed and intended to be used. Below is a list of commonly used markings.

T = Temporary Spare

Example: T145/70R17 106M If the tire size begins with a "T", it indicates that the tire is a "Temporary Spare". Temporary Spares are also called "Mini Spares" or "Space Savers". Temporary Spares are only designed to be used for a short period of time until the regular tire is repaired and/or replaced.

P = P-Metric

Example: P215/65R17 98T P-Metric tires were introduced in the late 1970s. P-Metric tires are the most commonly used type of tire on the market today. The P-Metric designation means they are designed primarily for use on passenger vehicles such as cars, minivans, light-duty pickup trucks, and sport utility vehicles. Light-duty pickup trucks are typically those with a ¼ ton or ½ ton load capacity.

Metric / Euro-Metric

Example: 185/65R15 88T Metric, or "Euro-Metric" tires have no leading letter designation. They are called Euro-Metric because the sizes originated in Europe. Euro-Metric sizes are equivalent to P-Metric sizes in the overall dimensions, but the Euro-Metric sizes have subtle differences in their load-carrying ratings and capabilities. Euro-Metric tires are typically used on European cars, but they are also frequently used on sport utility vehicles and vans.

LT - Light Truck (Prefix)

Example: LT235/75R15 104/101S/C Light Truck Metric tires sizes begin with the letters "LT". Tires with the "LT" designation are designed to be used on vehicles that are used to tow trailers or carry heavy loads. Vehicles in this range usually include sport utility vehicles, full-size vans, and medium-duty and heavy-duty pickup trucks with typically ¾ ton to 1-ton capacity.

LT - Light Truck (Suffix)

Example: 9.5-16.5LT 121/117R Example: 30x9.50R15LT 104S/C For light-duty, medium-duty, and heavy-duty pickup trucks (typically ½-ton, ¾-ton or 1-ton load capacity), sport utility vehicles, and vans. When a tire ends in the "LT" designation that means it falls into one of three light truck categories:

Numeric

Earlier numeric sizes are designed to carry heavy cargo loads and/or tow trailers.

Wide Base

Wide Base tires are also designed to carry heavy cargo loads and/or tow trailers on a wheel rim with a diameter of 16.5 inches.

Flotation

Flotation tires are wider, oversized tires designed to carry heavy cargo loads and/or tow trailers on loose surfaces such as sand, gravel, or dirt.

ST - Special Trailer Service

Example: ST175/80R13 Tires beginning with "ST" are special trailer tires and should only be used on car, boat, or utility trailers. "ST" tires are not designed for cars, vans, SUVs, or light trucks and should never be used on those types of vehicles.

C = Commercial

Example: 31x10.50R15/C 109R Euro-Metric tire sizes ending with a "C" designation are commercial tires designed to be used on delivery trucks and vans that are capable of carrying heavy loads. In addition to the "C" designation branding on the size, these sizes are also branded with a Load Range and Service Description rating (Load Range B, C, or D).

Section Width of Tire

Example: P225/45R17 91V The three leading digits following the Service Rating Prefix (if present) indicate the section cross-sectional width of the tire as measured in millimeters. In the example above, the tire's cross sectional width, measured from the widest point of its inner sidewall to the widest point of the outer sidewall when properly mounted on a specified wheel width is indicated at 225 millimeters. This measurement, also known as the "section width" can be easily converted from millimeters to inches by dividing the section width in millimeters by 25.4 mm/in as follows: (225 millimeters) / (25.4 mm/in) = 8.86 inches

Aspect Ratio of Sidewall

Example: P225/45R17 91V The two-digit number that usually follows the tire's section width measurement is known as the Aspect Ratio or Tire Profile measurement. In this example, the "45" indicates that the sidewall distance from the wheel rim to the outside of the tread is 45% of the section width. A lower Aspect Ratio indicates a lower-profile tire with a shorter sidewall, whereas a higher Aspect Ratio tire will have a taller sidewall and will look more like a doughnut. Because we know that the tire size shown in this example has a section width of 8.86 inches and the Aspect Ratio is 45%, the sidewall height for this tire will be 3.98 inches.

Tire and Wheel Diameter

Example: P225/45R17 91V Using our example tire size from above, the "17" indicates that both the tire and wheel should be matched to and should use a 17 inch diameter wheel. Tires usually come in the following widths (in inches): 8, 10, 12, 13, 14, 15, 17, 18, 19, 20, 22, 23, 24, 26, and 28. Tires with these diameters are typically found on most passenger cars, light-duty light trucks, SUVs, minivans, and vans. Tires with a rim diameter expressed in inches are called "Inch Rim" sizes.

Unique Wheel Diameters

In addition to the Inch Rim sizes, there are some unique sizes in the market today. Although not as common, tires can also be specified in half-inch diameters for some heavy-duty light trucks, box vans, and heavy-duty trailers. These sizes are usually 14.5, 15.5, 16.5, 17.5, and 19.5 inches, and an example would be 33x12.5R16.5 118R. Tires and wheels can also have their diameter expressed in millimeters and are known as "millimetric" sizes- specifically sized as 365, 390, and 415. Michelin originally introduced millimetric sizes in the 1970s and 1980s with their TRX tires that were used on many different car models. In North America, Michelin also introduced run-flat tires (integrated tire & wheel combinations) called the Michelin PAX System on a very limited basis as OE (Original Equipment). An example PAX System size of 245-680R460A 102V expresses tire and wheel dimensions as follows: The "A" in 460A signifies these tires feature "asymmetric" beads. The outside bead (450 mm) and inside bead (470 mm) are actually different diameters. Each of the "unique" diameter sizes for tires and wheels were developed to address some specific need in the industry, either because the tire and wheel design or the intended vehicle use dictated the unique sizing. The uniquely sized tire diameters have bead profiles that are different shapes from the traditional Inch Rim bead shapes. As a result, tires and wheels with unique rim diameters should never be combined with traditional Inch Rim tires and wheels. Prior to mounting tires on wheels, it is critical that the tire and wheel diameters are always confirmed to match.

Internal Construction

Example: P225/45R17 91V When a letter (B, D, or R) follows the two-digit Aspect Ratio, it is an indicator of the tire's construction. In this example, the "R" indicates that the tire has radial construction. Over 98% of all tires sold today are radial tires, where the internal body plies of the tire's construction radiate outward from the center. If there is a "D" instead of an "R", the tire has a "bias ply" construction, where the internal body plies of the tire crisscross on a diagonal pattern. Belted tires (specified with a "B") are practically non-existent at this time. However, the "B" indicates that the internal plies crisscross as in a "D" construction, but there is an extra layer of reinforcing belts under the tread area.

Speed Rating

Example: P225/45ZR18 92Y The tire's speed rating can also be indicated after the two-digit Aspect Ratio. However, since 1991, all other speed ratings are included in the Service Description. Today, the only speed rating today still included in the tire size is the "Z" rating.

Service Description Rating

Example: P225/45R17 91V Since 1991, the Service Description rating is mandatory for all speed ratings (except Z-rated tires) and appears at the end of the tire's size brand. The Service Description is used to identify the tire's Load Index and Speed Rating.

Please see the article Understanding Speed Ratings, Load Indexes, and Service Descriptions for more information on how to read the tire's Load Index and Speed Rating.

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