Tire Size Designations
Tire size designation is a crucial aspect of automotive maintenance and safety, yet it often remains a mystery to many drivers. Understanding the alphanumeric codes on the sidewall of a tire is essential for selecting the right tires, ensuring vehicle safety, and optimizing performance.
The tire size designation is typically presented as a sequence of numbers and letters, such as "P215/65R15." This code conveys critical information about the tire's type, width, aspect ratio, construction, and wheel diameter. Let's break down each component.
The first letter in the code indicates the tire type. For example, "P" stands for Passenger vehicle, "LT" denotes Light Truck, and "T" represents Temporary or spare tires. If no letter precedes the numbers, it usually indicates that the tire is a European metric tire, which is most commonly used in passenger vehicles.
Next is the three-digit number following the initial letter, representing the tire's width in millimeters from sidewall to sidewall. For instance, in "P215/65R15," the tire's width is 215 millimeters. This measurement affects the tire's contact with the road, influencing traction, handling, and fuel efficiency.
The number following the slash, known as the aspect ratio, indicates the height of the tire's sidewall as a percentage of its width. In our example, "65" means that the sidewall height is 65% of the tire's width. A lower aspect ratio often corresponds to a sportier performance tire with a shorter sidewall, enhancing handling but potentially reducing ride comfort.
The next part of the code is the letter "R," which denotes the tire's construction type. "R" stands for Radial, the most common type of tire construction, where the internal cords run radially across the tire. This design offers a smoother ride, better traction, and longer tread life compared to older bias-ply constructions.
Following the construction type is a two-digit number representing the wheel diameter in inches. In our example, "15" means that the tire is designed to fit a 15-inch wheel. It's crucial to match the tire's wheel diameter with the vehicle's wheels to ensure proper fitment and performance.
Beyond these primary components, tires also have additional markings that convey further information, such as the load index and speed rating. The load index is a numerical code that indicates the maximum load a tire can support when properly inflated, while the speed rating is a letter code that represents the maximum speed the tire can safely maintain. For example, a load index of 95 means the tire can support up to 1,521 pounds, and a speed rating of "H" indicates a maximum speed of 130 mph.
Tires may also include other designations, such as "M+S" for mud and snow, indicating the tire is suitable for all-season driving conditions, or "XL" for extra load capacity. These additional designations help drivers choose tires that meet their specific driving needs, whether for off-road adventures, winter driving, or high-performance sports cars.
Tire size may contain prefix and / or suffix letters as part of tire size designation to identify service conditions which may require different tire loads and inflation pressures.
Prefix Letters
Prefix letters come in front of tire size, like P225/55R15 or LT195/75R14.
AT - Indicates a tire intended for service on All-Terrain vehicles.
DH - Indicates a tire intended for agricultural and logging service (tire must be mounted on a DH drop center rim).
IF - Indicates an agricultural tire thay operates at 20% higher rated load than standard metric tires at the same inflation pressure.
K - Indicates a tire intended for 3-wheeled motorized vehicles without camber steering.
LT - Indicates a tire primarily intended for service on light trucks and pickups.
P - Indicates a tire primarily intended for service on passenger cars.
ST - Indicates a special tire for highway service trailers.
T - Indicates a tire intended for one-position temporary use as a spare only.
VA - Indicates a tire intended for agricultural and logging service (must be mounted on a VA multipiece rim).
VF - Indicates an agricultural tire that operates at 40% higher rated load than standard metric tires at the same inflation pressure.
Suffix Letters
Suffix letters come after tire size, like 33x12.50R15LT.
CFO - Indicates a Cyclic Field Operation tire intended for agricultural usage.
DT - Indicates a tire primarily designed for sand and paver service.
FI - Indicates an implement tire for agricultural towed highway service.
HC - Indicates a tire with a 17.5 rim diameter intended for service on low platform trailers.
IMP - Indicates a tire designed primarily for agricultural machines, implements or trailers which are not suitable for sustained high torque service.
IND - Indicates a drive wheel tractor tire used in industrial service.
K - Indicates a compactor tire for use on 5° drop center or semi-drop center rims having bead seats with nominal minus 0.032 diameter.
LT - Indicates a tire intended for service on Trucks, Busses, Trailers and Multipurpose Passenger Vehicles used in normal highway service.
M/C - Indicates tires and rims for certain motorcycles.
MH - Indicates a tire intended for service on Mobile Homes.
ML - Indicates a tire intended for Mining and Logging vehicles used in intermittent highway service.
NHS - Indicates a tire NOT intended for Highway Service.
SL - Indicates a tire with service limited to agricultural usage.
SS - Indicates a tire for off-highway vehicles such as mini and skid-steer loaders.
ST - Indicates a special tire for highway service trailers.
TG - Indicates an Off-the-Road tire for use on rims having bead seats with nominal +0.188" diameter (not for highway service).
TR - Indicates a tire for service on Trucks, Busses and other vehicles with rims having specified rim diameter of nominal plus 0.156" or plus 0.250".
Understanding tire size designation is more than just reading a code - it is about ensuring your vehicle performs optimally and safely under various conditions. By familiarizing yourself with these markings, you can make informed decisions when purchasing new tires, leading to improved safety, better handling, and enhanced fuel efficiency. Proper tire selection, based on accurate size designation, ensures that your vehicle operates at its best, providing a smooth and safe driving experience.
All listed guides, data and/or calculations are for informational purposes only. TirePressure.com does not warrant or make any representations regarding the accuracy of or the results of the use of this information.