P-Metric
It allows higher maximum inflation pressures and is based on the metric system. Rim diameters are expressed in inches. P-Metric tires have maximum inflation pressures of 35, 44, or 50 psi. Today, most tires include the load index and speed rating symbols, which are referred to collectively as the service description.
To understand how this system works, consider these three P-Metric tire sizes: P225/60VR15, P225/60R15, P225/60R15 96V
P – passenger-car tire designation
225 – section width, in millimeters
60 – aspect ratio
R – radial construction
15 – rim diameter, in inches
96V – service description (a combination of load index and speed rating)
V – speed rating
Note: Not all elements are present in all size designations. Some tires may not be speed rated, while others may not have a service description. P-Metric designation sizes are based on standards from the Tire and Rim Association (TRA).
| 1 | P – | passenger-car tire designation |
| 2 | 215 – | section width, in millimeters |
| 3 | 60 – | aspect ratio |
| 4 | R – | radial construction |
| 5 | 16 – | rim diameter, in inches |
| 6 | 94T – | service description (a combination of load index and speed rating) |
| T – | speed rating |
Metric
The Metric system simply represents a conversion of the old Numeric system from inches to millimeters, and often looks very similar to the P-Metric system. In fact, much of the information is exactly the same. If an aspect ratio does not appear in a size designation, it is assumed to be 82.
To understand how this system works, consider these three Metric sizes: 185R14, 185SR14, 185/70SR14
185 – section width, in millimeters
70 – aspect ratio
S – speed rating
R – radial construction
14 – rim diameter, in inches
| 1 | 215 – | section width, in millimeters |
| 2 | 60 – | aspect ratio |
| 3 | R – | radial construction |
| 4 | 16 – | rim diameter, in inches |
ISO Metric
The ISO (International Standards Organization) Metric system is an expansion of the Metric system. It combines the information from the Metric system with load index and speed rating. Today, most tire sizes include load index and speed rating symbols, which are referred to collectively as the service description.
To understand how this system works, consider this ISO Metric size: 185/70R14 87S
185 – section width, in millimeters
70 – aspect ratio
R – radial construction
14 – rim diameter, in inches
87S – service description
S – speed rating
Note: ISO Metric designation sizes are based on standards from the European Tyre and Rim Technical Organization (ETRTO). These standards are NOT interchangeable with those from the Tire and Rim Association (TRA) for P-Metric tires.
| 1 | 255 – | section width, in millimeters |
| 2 | 35 – | aspect ratio |
| 3 | ZR – | radial construction – speed rating |
| 4 | 18 – | rim diameter, in inches |
| 5 | 90W – | service description (a combination of load index and speed rating) |
| W – | speed rating – refined |
Size Designations: Light Trucks. Three sizing systems are in use for light-truck tires today. They are: Metric, High Flotation, and Numeric.
LT-Metric
The Metric system is the light-truck version of the P-Metric system. It is sometimes called the LT Metric system because all sizes begin with the letters “LT”.
To understand how this system works, consider this Metric size: LT 245/75R16 E
LT – light truck designation
245 – section width, in millimeters
75 – aspect ratio
R – radial construction
16 – rim diameter, in inches
E – load range
| 1 | LT – | light truck tire designation |
| 2 | 325 – | section width, in millimeters |
| 3 | 50 – | aspect ratio |
| 4 | R – | radial construction |
| 5 | 22 – | rim diameter, in inches |
| 6 | E – | load range |
High Flotation
All High Flotation sizes begin with a two-digit number that indicates the tire’s overall diameter in inches.
To understand how this system works, consider this High Flotation size: 31×10.50R15LT C
31 – overall diameter, in inches
10.50 – section width, in inches
R – radial construction
15 – rim diameter, in inches
LT – light-truck designation
C – load range
| 1 | 37 – | overall diameter, in inches |
| 2 | 13.50 – | section width, in inches |
| 3 | R – | radial construction |
| 4 | 17 – | rim diameter, in inches |
| 5 | LT – | light truck designation |