What Is Trailer Tire Load Index?
A trailer tire load index refers to how much weight a tire can safely support when it is properly inflated.
When choosing new tires or replacing old ones, matching the proper trailer tire load index to your trailer’s gross weight ensures you’re not exceeding the tire’s structural limits. Exceeding that threshold, even slightly, can cause overheating, tread separation, or blowouts at highway speeds.
Where to Find It on the Sidewall
The trailer tire load index is found on the sidewall, usually after the tire size and load range. You can cross-reference that number with a load index chart to find its weight rating. At times you will see two load indices listed. In these cases, it will show you single-use followed by dual-use limits.
Check this number when buying replacement tires, as even small variances in the trailer tire load index value can change the total load capacity by hundreds of pounds.
Load Index vs. Load Range
There are all sorts of different specs for trailer tires, including the load index and load range. The index tells you the capacity of each tire, while the range measures the maximum PSI tolerance and internal strength.
Example Decoding
For example, if you have a tandem-axle cargo trailer with four tires and a gross vehicle weight rating of 9,000 pounds, you’ll need four tires that provide at least 2,250 pounds of support each. If you have tires with a load index of 111, they will provide about 2,400 pounds each, giving a small safety buffer. If using duals, you want to be sure to use the second load index shown, which will be a smaller number.
However, upgrading to a Load Range E with a load index of 113 gives you an even larger margin. Choose the combination that covers your actual weight, not just the trailer’s empty weight. That way, you have enough cushion for fuel, cargo, and accessories that count toward the total capacity.
How to Calculate Your Required Per-Tire Capacity
To crunch the numbers and determine how much capacity you need for each tire, follow these simple steps.
GVWR, GAWR, and Tongue Weight (10-15%)
Start by locating your trailer's GVWR and GAWR. Subtract 10-15% of the GVWR to account for tongue weight, which is passed to your vehicle’s hitch. Divide the remaining weight by the number of tires.
Imagine your GVWR is 8,000 pounds, and 800-1,200 pounds rest on the hitch. Each of your four tires should handle roughly 1,700-1,800 pounds. Choose a trailer tire load index that matches or exceeds that number.
Single vs. Tandem/Tridem Axles and Uneven Load in Turns
Trailers with multiple axles don’t share weight perfectly evenly, especially in tight turns or on uneven pavement. Inner tires often carry more stress during cornering. When calculating tire needs, you should not assume that all tires share the load equally. That’s why experienced haulers add a margin of 10-15% to their calculated requirements.
Wheel PSI Limits and Matching Tire/Wheel Ratings
You must also consider the inflation pressure. Inflating beyond the wheel’s rated PSI can damage the rim or cause catastrophic failure. Carefully verify that your wheel is rated for the same PSI as your tire’s maximum cold inflation.
Build a 10-20% Capacity Reserve (Heat, Road Shock, Scrub)
Aim for a 10-20% capacity reserve above your maximum loaded weight. This saves room for variables like heat buildup, road shock, and tire scrub when making tight turns. A small reserve ensures you don’t exceed the load index rating.
What Is a Trailer Tire Speed Rating?
The trailer tire speed rating is a letter designation that represents the maximum safe speed a tire can sustain under its rated load. Most special trailer (ST) tires have ratings between J and N, which corresponds to speed thresholds between 62-87 MPH.
Knowing your trailer tire speed rating helps you set safe towing limits that protect against excessive heat and wear.
Common Trailer Symbols J-N
The trailer tire speed ratings you’ll encounter most often are J (62 mph), L (75 mph), M (81 mph), or N (87 mph). Upgrading to a higher trailer tire speed rating means the tire can better manage heat at highway speeds.
How Speed Rating Relates to Load and PSI
Your trailer tire speed rating and load index are linked. The faster you drive, the more flex and heat your tire will experience, which effectively reduces its weight-carrying capacity. Follow the manufacturer's load/pressure charts to stay safe, especially for ST tires.
Heat Management at Highway Speeds
Heat is the silent killer of trailer tires. Sustained driving above the trailer tire speed rating causes rubber compounds to soften and sidewalls to degrade. With this in mind, stay within the trailer tire speed rating and make sure your tires are properly inflated to avoid these issues. Please note, it is a general recommendation not to exceed 55-65 mph when towing a trailer, regardless of the tire’s speed rating.
Why Load Index and Speed Rating Must Be Chosen Together
Your trailer tire load index and trailer tire speed rating work together. A tire that can carry your load but isn’t rated for your driving speed may fail from heat stress. Likewise, a tire rated for higher speeds with insufficient load capacity will deform under pressure.
Trailer Tire Selection Walkthrough by Use Case
Boat Trailers
Boat trailers will be exposed to moisture and face ramp traction issues. Choose tires with a strong load index and a speed rating high enough to handle highway driving.
Cargo/Enclosed & Car Haulers
A mid-to-high trailer tire load index and an M or N speed rating are ideal so you can handle heavier loads and cruise along the highway safely.
RV/Travel Trailers
RV tires must endure extended parking and long drives. Make sure you have a high-load radial tire and ensure the load index meets your GVWR.
Equipment & Dump Trailers
You’ll need tires with high load index ratings for heavy-duty equipment such as dump trailers.
Inflation and Maintenance Essentials
Cold PSI Checks and Load Tables (Many ST Ratings at Max Cold PSI)
Check your tire pressure when the tires are cold. Once you start driving, the friction and heat can raise the PSI.
Pre-Trip Inspection, Lug Torque, Balancing
Before heading out on the road, set aside time to check your tread depth, valve stems, and the torque of your lug nuts. These precautions can extend your tread life and preserve the accuracy of your trailer tire speed rating.
Rotating, Age/Replacement, Storage Practices
Rotate tires every 3,000–5,000 miles, replacing them every five to six years regardless of tread. Don’t forget that UV exposure and dry rot can also make your tires more susceptible to failure.
Safety Margins and Best Practices
Keep a 10–20% Load Reserve
Allow a 10–20% load reserve to prevent overloading during turns, bumps, or thermal expansion.
Avoid Mixing Constructions, Sizes, or Load Ranges
All of your tires should match in terms of the load index and speed rating. For this reason, it’s a good idea to replace them in complete sets.
Post-Install Re-Torque and First-Trip Recheck
After installing new tires, re-torque the lugs after the first 50–100 miles. Check the PSI again before your next trip.
Safe Towing Starts With the Right Tires
When shopping for trailer tires, use the tips above to choose safe, effective, and reliable options. If you still have questions, Tires4That can answer them and help you find the best tires for your trailer.