Understanding how tire sizes work involves deciphering the series of letters and numbers printed on the sidewall of your tire. This code provides essential information about the tire's dimensions, construction, and performance capabilities.
A standard tire size marking, often referred to as the P-metric system, looks like a sequence such as P215/60R16 94H. Each part of this code tells you something specific about the tire.
Here's a breakdown of the common components:
Service Type (P)
- The first letter indicates the tire's intended use or service type.
- P: Stands for "Passenger" vehicle tire. This is the most common type for cars, SUVs, and light pickup trucks.
- LT: Stands for "Light Truck" tire. Used for larger SUVs, vans, and heavy-duty pickup trucks.
- ST: Stands for "Special Trailer" tire. Designed for trailers.
- T: Stands for "Temporary" spare tire.
Tire Width (215)
- The first number after the service type (e.g., 215) represents the tire's width in millimeters.
- This is the measurement from the widest point of the outer sidewall to the widest point of the inner sidewall when the tire is mounted on a specified wheel.
Aspect Ratio (60)
- After tire width, you'll find a slash and a two digit number. This indicates the aspect ratio, which gives the tire height.
- In our example, 60 is the aspect ratio.
- Aspect ratio specifically is the tire sidewall height as a percentage of the width. A high aspect ratio means that a tire has a larger sidewall while a lower one means a lower sidewall.
- So, an aspect ratio of 60 means the sidewall height is 60% of the tire width (60% of 215 mm). A lower number (like 40 or 50) means a shorter, lower-profile sidewall, often found on sports cars.
Construction (R)
- The letter after the aspect ratio indicates how the tire was constructed.
- R: Stands for "Radial," which is the most common tire construction today. Layers of cord run radially across the tire.
- D: Stands for "Diagonal" or "Bias Ply." Layers of cord run diagonally. Less common on modern passenger vehicles.
- B: Stands for "Belted." Similar to bias ply but with reinforcing belts.
Wheel Diameter (16)
- The number after the construction type (e.g., 16) indicates the diameter of the wheel that the tire is designed to fit.
- This measurement is given in inches.
- So, a tire marked "16" is designed to fit a 16-inch diameter wheel.
Load Index (94)
- The two or three-digit number after the wheel diameter (e.g., 94) is the tire's load index.
- This is a code that corresponds to the maximum weight the tire can carry when properly inflated.
- A higher number indicates a higher load capacity. You need to consult a load index chart to find the specific weight capacity (e.g., 94 typically corresponds to 1,477 lbs or 670 kg).
Speed Rating (H)
- The final letter (e.g., H) indicates the maximum speed at which the tire can safely carry the specified load under optimal conditions.
- Like the load index, this is a code that corresponds to a specific speed in mph or km/h.
- Common speed ratings include:
- S: 112 mph (180 km/h)
- T: 118 mph (190 km/h)
- H: 130 mph (210 km/h)
- V: 149 mph (240 km/h)
- W: 168 mph (270 km/h)
- Y: 186 mph (300 km/h)
Example Breakdown Table
Let's summarize our example P215/60R16 94H:
| Code Part | Meaning | Value in Example | Explanation |
|---|---|---|---|
| P | Service Type | P | Passenger Vehicle Tire |
| 215 | Tire Width | 215 | 215 millimeters wide |
| /60 | Aspect Ratio | 60 | Sidewall height is 60% of the width |
| R | Construction | R | Radial construction |
| 16 | Wheel Diameter | 16 | Fits a 16-inch diameter wheel |
| 94 | Load Index | 94 | Indicates maximum load capacity (refer to chart - e.g., 1,477 lbs) |
| H | Speed Rating | H | Indicates maximum speed capacity (refer to chart - e.g., 130 mph) |
Other Markings
Beyond the main size code, you might also find:
- DOT Code: A series of letters and numbers indicating the tire's manufacturing plant, size code, and date of manufacture (last four digits: week/year).
- Treadwear, Traction, Temperature Ratings (UTQG): Ratings indicating the tire's expected wear rate, ability to stop on wet pavement, and resistance to heat.
- Max. Inflation Pressure: The maximum cold inflation pressure allowed for the tire.
Understanding these markings ensures you select the correct tire size for your vehicle, crucial for performance, safety, and proper fitment.
