Trailer tire numbers are a coded language providing crucial information about the tire’s size, load capacity, speed rating, and construction. Understanding these numbers is essential for selecting the correct tires for your trailer, ensuring safe and efficient towing.
Understanding the Alpha-Numeric Code
The markings on your trailer tire sidewall aren’t just random characters; they’re a standardized system designed to convey vital information. Let’s break down each component:
1. Tire Type (Optional)
The first letter, often but not always present, indicates the tire’s intended use. Common designations include:
- P: Passenger Vehicle – Typically not used on trailers but occasionally found on very light utility trailers.
- LT: Light Truck – Generally suitable for some utility and boat trailers, offering increased load capacity compared to passenger tires.
- ST: Special Trailer – Designed specifically for trailers, these tires have reinforced sidewalls and are engineered to handle the unique stresses of towing. ST tires are the recommended choice for most trailers.
2. Tire Width
The next number represents the tire’s section width in millimeters. This is the distance from sidewall to sidewall at the tire’s widest point when properly inflated. For example, a tire marked ‘205’ has a section width of 205 millimeters.
3. Aspect Ratio
Following the width is a slash (/) and then a two-digit number representing the aspect ratio. This is the percentage of the section width compared to the tire’s sidewall height. For instance, an aspect ratio of ’75’ means the sidewall height is 75% of the section width. A lower aspect ratio generally indicates a shorter sidewall.
4. Construction Type
The letter following the aspect ratio indicates the tire’s internal construction.
- R: Radial – The most common construction type, with plies running radially across the tire. Radial tires generally offer better ride quality and fuel efficiency.
- D: Bias-Ply – Plies run diagonally across the tire. Bias-ply tires are typically more durable and resistant to punctures, making them suitable for heavy-duty applications but often resulting in a stiffer ride.
5. Rim Diameter
The next number is the rim diameter in inches. This number must match the rim size of your trailer’s wheels. For example, a tire marked ’14’ is designed for a 14-inch rim.
6. Load Index
The load index is a numerical code indicating the maximum weight a single tire can carry when properly inflated. This number isn’t a direct weight measurement; you need to consult a load index chart to find the corresponding weight in pounds or kilograms. Always ensure the tires’ combined load capacity exceeds the trailer’s maximum gross weight rating (GVWR).
7. Speed Rating
The final letter is the speed rating, indicating the maximum speed the tire is designed to sustain for extended periods. Exceeding the speed rating can cause tire failure. Trailer tires often have speed ratings like “L” (75 mph), “M” (81 mph), or “N” (87 mph). It’s crucial to adhere to the speed rating for safe towing.
Beyond the Basics: Understanding Additional Markings
Besides the core numbers, your trailer tire might contain other useful information:
- DOT Code (Department of Transportation): Indicates the tire meets safety standards and provides the week and year of manufacture. This is helpful for determining the tire’s age.
- “Max Load” and “Max Pressure”: Indicate the maximum weight the tire can carry and the maximum air pressure required to achieve that load capacity, respectively.
- “Treadwear,” “Traction,” and “Temperature” Ratings: Found on some trailer tires, these provide relative indicators of tire performance in those areas.
FAQs: Deepening Your Understanding of Trailer Tire Numbers
Here are answers to some frequently asked questions about trailer tire numbers:
1. Why are ST tires recommended for trailers?
ST tires are specifically designed for trailer applications. They have reinforced sidewalls to handle the lateral forces and swaying often encountered when towing. They also typically have a higher load capacity than passenger (P) tires of the same size. This added strength and stability are crucial for safe and reliable towing.
2. How do I find the correct load index for my trailer?
First, determine your trailer’s Gross Vehicle Weight Rating (GVWR). This is the maximum permissible weight of the trailer, including the weight of the trailer itself and its cargo. Divide the GVWR by the number of tires on the trailer (usually 2 or 4) to determine the minimum load capacity required per tire. Then, consult a load index chart to find the corresponding load index number. Always round up to the next highest load index if your calculation falls between two values.
3. What happens if I overload my trailer tires?
Overloading trailer tires can lead to premature tire wear, tire failure, and even accidents. Overloaded tires generate excessive heat, which can weaken the tire’s internal structure and cause it to blow out. It also reduces braking effectiveness and handling stability.
4. Can I use passenger (P) tires on my trailer?
While technically possible on very light utility trailers with low GVWR, it’s generally not recommended. P tires are not designed for the stresses of towing and lack the reinforced sidewalls of ST tires. Using P tires on a trailer can compromise safety and lead to tire failure.
5. What is the importance of checking the DOT code on my trailer tires?
The DOT code indicates the tire’s age. Over time, tires can deteriorate, even if they haven’t been used much. Sunlight, ozone, and other environmental factors can cause the rubber to dry out and crack, reducing its strength and increasing the risk of failure. A general rule of thumb is to replace trailer tires every 5-7 years, regardless of tread depth.
6. What does “cold tire pressure” mean, and why is it important?
“Cold tire pressure” refers to the tire pressure measured when the tires are cold, meaning they haven’t been driven on recently. Checking and adjusting tire pressure when the tires are cold provides the most accurate reading and ensures proper inflation. Proper inflation is crucial for maximizing tire life, fuel efficiency, and safety.
7. How often should I check my trailer tire pressure?
It’s recommended to check your trailer tire pressure before each trip, especially on longer journeys. Tire pressure can fluctuate with temperature changes, so it’s essential to ensure they are properly inflated to the recommended pressure specified on the tire sidewall.
8. Can I use different tire sizes on the same trailer axle?
No. All tires on the same axle must be the same size and type. Using different tire sizes can cause uneven wear, stress on the axle, and handling problems. It’s critical for consistent and predictable trailer behavior.
9. What if I can’t find ST tires in the size I need?
If you can’t find ST tires in the exact size you need, consult with a qualified tire professional. They can help you find an appropriate substitute that meets the load and speed requirements of your trailer. Light Truck (LT) tires might be an option, but you need to ensure they meet or exceed the load capacity of the original ST tires.
10. What is “ply rating,” and how does it relate to load capacity?
Ply rating indicates the tire’s strength and load-carrying capacity. While modern tires don’t necessarily use the same number of actual plies, the ply rating represents the equivalent strength of a tire with that number of plies. Higher ply ratings generally indicate a stronger tire with a higher load capacity. Look for the load index for precise load information.
11. Where can I find a load index chart?
Load index charts are readily available online and in most tire shops. A simple search for “load index chart” will provide numerous options. Many tire manufacturers also provide load index charts on their websites.
12. What are the signs that my trailer tires need to be replaced?
Besides age, other signs that your trailer tires need replacing include:
- Visible tread wear below the legal minimum.
- Cracks or bulges in the sidewall.
- Uneven wear patterns.
- Punctures or repairs.
If you notice any of these signs, it’s essential to replace your tires immediately to ensure safe towing. Remember, proactively maintaining your trailer tires is an investment in your safety and the longevity of your equipment.