P-Metric System
The P-metric system is used internationally and requires both English and metric measurements to describe tire size, construction and performance. This designation is employed on tires for newer vehicles. The designation is composed of three letters and three numbers to signify (in this order):
- Tire type -- describes tire use
- P for passenger
- C for commercial
- T for temporary
- LT for light truck
- Tire section width -- in millimeters (1"=25.4 mm); not tread width, but sidewall-to-sidewall width
- Aspect ratio (profile) -- tire section height divided by tire section width; essentially, tire height as a percentage of tire width
- Speed rating -- indicates maximum permissible tire speed
- S for 112 mph maximum
- T for 118 mph maximum
- U for 124 mph maximum
- H for 130 mph maximum
- V for over 130 mph
- Z for over 149 mph
- W for over 168 mph
- Construction type -- basic category
- R for radial
- B for bias-belted
- D for bias ply/diagonal
- Wheel (or rim) diameter -- stated in inches
The above information is found on the tire sidewall. For example, a P235/60VR15 tire would be a passenger car tire 235 millimeters wide (sidewall-to-sidewall) with 60% profile and maximum tire speed of 165 mph using radial construction for a 15" diameter wheel.
NOTE: Some tires may have a separate Service Description instead of a speed rating. The Service Description would be composed of a load index and speed symbol. If a tire has a "V" speed rating in the Service Description, the maximum limit is 149 mph. Speed ratings do not and should not be interpreted to mean the vehicle can be safely driven at that speed. Most roads, traffic conditions, weather, vehicle traits and driver reflexes all support driving at or below the legal speed limit.
Additionally, the sidewall would state the
- maximum load (in lb and kg)
- maximum pressure (in psi and kPa)
- tire construction (type of cord and number of plies)
- DOT (Department of Transportation) compliance
- mud and snow use (M+S label), if applicable