Tire Compendium

Welcome to the Tire Compendium, an exclusive resource for industry professionals like you—tire shop employees and experts. From tire basics to types, specifications, and fitment guidance, this comprehensive guide will elevate your expertise. Decode sidewall codes and offer tailored recommendations, optimizing vehicle performance and safety. Let's raise the bar together in the tire industry!

Tire Size

Understanding tire size is a critical aspect of your role as a wheel and tire shop professional. The numbers on the sidewall may seem complex, encompassing both millimeters and inches, but it's really quite simple once you understand the basics. Furthermore, the correct size for a customer's car, truck, or trailer may vary based on driving conditions and individual preferences.

As a starting point for any tire-related service, it's essential for wheel and tire shop professionals to accurately understand the Original Equipment (OE) fitment for a customer's vehicle. This information can typically be found in the owner's manual or on the door placard, usually situated on the driver's side door jam. This size signifies the manufacturer's intended specifications for the vehicle and serves as a foundational reference for understanding the OE tire characteristics.

However, it's also vital to recognize that many factors may necessitate changes to the original tire size. These factors can include the installation of aftermarket wheels, modifications to the vehicle's suspension, and specific use cases outlined by the customer. Navigating these considerations with expertise ensures that you can provide tailored solutions that align with both the vehicle's design and the customer's unique needs.

How to read tire sizes

Tire sizes are more than just a series of numbers and letters printed on a tire's sidewall. They are crucial codes that reveal essential information about a tire's dimensions, type, and performance characteristics. For professionals working in wheel and tire shops, the ability to accurately read and interpret these codes is fundamental to providing customers with the right products and services. Understanding tire sizes ensures proper fitment, alignment, and optimal vehicle performance, and it helps in maintaining safety standards.

When it comes to tire sizing, there are several types, but the two most common are known as Metric and Flotation.

Metric Sizing

This is the standard used by most passenger vehicles around the world. It includes details such as the tire's width, aspect ratio, construction, and diameter, among other specifications. A typical example of a Metric size would be 225/55R17.

How to Read Metric Tire Sizes

Flotation Sizing

Flotation sizes are typically used for trucks and off-road vehicles. These sizes are often displayed differently from Metric sizes and include information relevant to the tire's application on heavier or specialized vehicles. An example might be 31x10.50R15LT.

How to Read Flotation Tire Sizes

Tire Types

The tire type, often indicated by a letter or letters at the beginning of the tire size description, plays a crucial role in identifying the specific application or vehicle classification for which a tire is designed. Recognizing and understanding these types is essential for professionals in wheel and tire shops as it ensures the appropriate selection and installation of tires, aligned with the vehicle's purpose and the customer's needs.

Passenger (P)

P-metric tires are designed for passenger vehicles, including cars, minivans, SUVs, and light-duty pickup trucks. Selecting a P-type tire means adhering to standard measurements and specifications for typical passenger vehicles. It ensures comfort, performance, and safety in everyday driving conditions.

Light Truck (LT)

LT tires are constructed for light to medium trucks, larger SUVs, and vans. They are built to handle heavier loads and more rugged conditions compared to P-type tires. Knowing when to choose an LT tire is vital for vehicles that require enhanced durability and can handle heavy-duty applications, such as towing or off-road driving.

Special Trailer (ST)

ST tires are specifically designed for trailers, including boat trailers, utility trailers, and campers. Utilizing ST tires ensures that the unique needs of trailers, such as load distribution and stability, are met. Incorrect tire types can lead to poor handling and reduced safety.

Temporary (T)

T-type or temporary tires are commonly known as spare or "donut" tires. They are intended for short-term use to replace a flat or damaged tire. Recognizing a T-type tire ensures that it is used appropriately as a temporary solution, not for prolonged driving, as it may not provide the same performance or safety features as a standard tire.

Section Width

The section width of a tire refers to the measurement of the tire's width from one sidewall to the other. It's one of the most fundamental aspects of a tire's size and plays a significant role in how a tire fits and performs on a vehicle.

Section Width Diagram

The section width is typically measured in millimeters and is part of the standardized code found on a tire's sidewall. For instance, in the tire size 225/55R17, the section width is 225 millimeters. This measurement doesn't include any protruding features like sidewall lettering or protective ribs, focusing solely on the core width of the tire.

Aspect Ratio

The aspect ratio is a crucial component in understanding tire sizing. It represents the height of the tire's cross-section (or sidewall) relative to its width. In other words, it's the ratio of the tire's height to its section width, expressed as a percentage. The aspect ratio is typically found in the tire size description on the sidewall.

Aspect Ratio Diagram

For example, in the tire size 225/55R17, the aspect ratio is 55, meaning the height of the tire's sidewall is 55% of its section width. In this example the sidewall height would be 123.75 MM (~4.8 inches). This can be useful in many cases, especially when changing the diameter of the wheel and trying to maintain the same outside diameter of the tire. You will need to adjust the aspect ratio to compensate for the larger wheel diameter.

Tire Construction

Tire construction refers to the arrangement and material of the internal structure of a tire. It directly impacts the tire's overall performance, durability, and suitability for various applications. Among the different construction types, radial (R) and diagonal (bias) (D) constructions are the most common, each offering distinct characteristics and benefits.

Radial Construction (R)

Radial construction is characterized by the radial alignment of internal cords or plies. In this design, the fibers run perpendicularly to the tire's direction of travel, or radially from the center of the tire. This leads to improved ride comfort due to more flexible sidewalls and enhanced tread life through more uniform wear. Reduced rolling resistance in radial tires can also contribute to fuel economy. Radial tires have become the standard for most passenger vehicles, light trucks, and commercial vehicles because of their numerous advantages in comfort and efficiency.

Diagonal (Bias) Construction (D)

Diagonal (Bias) construction, on the other hand, features internal plies that are laid diagonally to the tire's direction of travel, with the plies overlapping in a crisscross pattern. This construction type offers greater load capacity, making it capable of handling heavier loads, and is known for its toughness and durability, particularly on rough or off-road surfaces. However, diagonal tires may not provide as smooth a ride as radial tires and are generally less fuel-efficient. They are often used in specific applications like agricultural equipment, off-road vehicles, and certain trailers where their strength is advantageous.

Run-Flat Tires (F)

The designation "F" in modern tire construction typically refers to run-flat tires. These tires are designed to maintain their shape and continue to perform even when they have lost all air pressure. The ability to run flat for a certain distance at a reduced speed provides added safety and convenience in the event of a puncture or blowout, allowing drivers to reach a safe location or service center for repair or replacement. Understanding and recognizing the "F" designation is essential for professionals, as run-flat tires require specific handling, installation, and compatibility considerations.

Inside Diameter

The inside diameter, commonly referred to as the wheel diameter, refers to the distance across the inside of the tire in inches or millimeters, depending on the region and standard. This measurement corresponds directly to the diameter of the wheel on which the tire is mounted.

Wheel diameters vary widely across different types of vehicles, from small passenger cars to heavy-duty trucks. Common wheel diameters include, but are not limited to:

  • 13 to 16 inches: Typically found in compact and some mid-size cars.
  • 17 to 20 inches: Common in larger cars, SUVs, and light trucks.
  • 21 inches and above: Often used in luxury vehicles, performance cars, and specialized applications.

Load Index

When it comes to selecting the right tire for a vehicle, one critical factor that must be taken into consideration is the Load Index. This guide aims to equip you, the tire professional, with a comprehensive understanding of what the load index means, why it's important, and how to apply this knowledge in various scenarios, including dually applications.

The load index is a numerical value that corresponds to the maximum weight a tire can carry when inflated properly. This figure is not just a recommendation; it's a safety standard that must be met or exceeded. Selecting a tire with an inappropriate load index can lead to tire failure and safety hazards.

In some commercial and heavy-duty applications, such as dually wheels (where two tires are paired on each side), a tire may display two load indexes. The first number refers to the load capacity when used in a single configuration, while the second number refers to the load capacity when used in a dual configuration.

CodeWeight (Lbs)Weight (KG)
65639290
66661300
67677307
68694315
69717325
70739335
71761345
72783355
73805365
74827375
75853387
76882400
77908412
78937425
79963437
80992450
811019462
821047475
831074487
841102500
851135515
861168530
871202545
881235560
891279580
901323600
911356615
921389630
931433650
941477670
951521690
961565710
971609730
981653750
991709775
1001764800
1011819825
1021874850
1031929875
1041984900
1052039925
1062094950
1072150975
10822051000
10922711030
11023371060
11124031090
11224691120
CodeWeight (Lbs)Weight (KG)
11325351150
11426011180
11526791215
11627561250
11728331285
11829101320
11929981360
12030861400
12131971450
12233071500
12334171550
12435271600
12536381650
12637481700
12738581750
12839681800
12940791850
13041891900
13142991950
13244092000
13345422060
13446742120
13548062180
13649382240
13750712300
13852032360
13953572430
14055122500
14156772575
14258422650
14360082725
14461732800
14563932900
14666143000
14767793075
14869453150
14971653250
15073853350
15176063450
15278263550
15380473650
15482673750
15585433875
15688184000
15790944125
15893704250
15996454375
16099214500

Speed Rating

The speed rating is a standardized system that classifies a tire's capability to handle sustained speeds without exceeding its design limitations. It is denoted by a letter code, each representing a specific maximum speed. The speed rating ensures that the tire's performance, safety, and durability align with the vehicle's top speed. The speed rating is usually found at the end of the tire's size designation, right after the load index. For example, in the code 225/45R17 94W, the letter "W" denotes the speed rating.

In addition to the standard speed ratings, there are special designations that professionals must be aware of when dealing with specific applications or unique vehicle requirements. For instance, some high-performance tires may carry a "Y" rating, indicating a maximum speed of up to 186 mph (300 km/h), while an additional "ZR" rating can signify capabilities beyond certain thresholds. Another example is the "C" rating, commonly found in commercial vehicle tires, which often relates to the tire's capacity for carrying loads at certain speeds. Understanding these unique designations and their implications is essential in the professional selection and fitting of tires, as they directly impact the performance, safety, and compliance of the tire with specific vehicle needs.

CodeSpeed Rating (MPH)Speed Rating (KPH)
A135
A2610
A3915
A41220
A51625
A61930
A72235
A82540
B3150
C3760
D4065
E4370
F5080
G5690
J62100
K68110
CodeSpeed Rating (MPH)Speed Rating (KPH)
L75120
M81130
N87140
P94150
Q100160
R106170
S112180
T118190
U124200
H130210
V149240
Z149240
W168270
(W)168270
Y186300
(Y)186300

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