Trailer Tire Load Index: How to Match GVWR

What does the load index mean?

The load index is a numeric code molded into the sidewall that corresponds to the tire's maximum load rating at the inflation pressure shown on the sidewall. Federal regulations define load rating as the maximum load a tire is rated to carry for a given inflation pressure (49 CFR 571.139). For trailers, you will typically see the load index as part of the service description, such as a three-digit number after the size.

Load index is different from load range, which is a letter (C, D, E, F) that indicates ply rating and maximum inflation pressure. Both matter for trailers: the load index determines the load capacity at a specific pressure, while the load range sets how high you can inflate. A tire with a higher load index can carry more weight, but only if you run enough pressure. Learn more about load index ratings.

The federal rule: match tire capacity to axle rating

Federal Motor Vehicle Safety Standard 571.110, which applies to trailers with a GVWR of 10,000 pounds or less, requires that the sum of the maximum load ratings of the tires fitted to an axle be not less than the gross axle weight rating (GAWR) of that axle system. For heavier trailers, Standard 571.120 has the same requirement. This is a legal minimum; you cannot safely load a trailer beyond the capacity of its tires.

That means you need to know the GAWR for each axle, not just the total GVWR. On a tandem-axle trailer, the front and rear axles may have different ratings. The certification label on the trailer, usually near the tongue or on the frame, lists these values. If the label is missing, weigh the trailer at a certified scale to estimate the axle loads. See how tire load ratings interact with vehicle weight.

How to calculate the load index you need

Start with the highest GAWR on the trailer. Divide that number by the number of tires on that axle. For a single-axle trailer with two tires, divide by two; for a tandem axle with four tires, each axle carries its own GAWR, so divide each by two. The result is the minimum load capacity each tire on that axle must provide.

Once you have that number, look up the load index on a manufacturer's chart to find a tire whose maximum load rating at the recommended cold inflation pressure is at least that value. Many trailer tires have load index numbers in the range of 80 to 120, but the exact number depends on the tire size and construction. Always verify the load rating on the sidewall or in the tire's spec sheet, because the load index alone does not tell you the load rating unless you know the pressure. Refer to a load capacity chart for a quick reference.

For trailer service, ST (special trailer) tires are built to FMVSS 119, which sets performance and marking requirements for tires used on trailers. ST tires have a different load formula than passenger car tires, and they are designed to handle the constant load and heat of trailer operation. Do not use a passenger car tire on a trailer unless the trailer manufacturer allows it. Compare ST and LT tires for trailers.

Load range vs. load index

Load range letters (C, D, E, F) indicate the ply rating and the maximum inflation pressure of the tire. A higher load range allows you to run higher pressure, which in turn increases the load capacity at that pressure. The load index is the numeric code that maps to a specific maximum load at a given pressure. They are complementary: you need a load range that supports the inflation pressure you plan to use, and a load index that covers the weight.

For example, a load range D tire might have a maximum pressure of 65 psi, while a load range E tire might go to 80 psi. The actual load you can carry depends on both the inflation pressure and the load index. Always follow the trailer manufacturer's recommended inflation pressure, which is based on the tire size and load. If you are unsure, compare load range C vs. D or D vs. E to see which is appropriate for your trailer. Learn more about load range designations and how load range affects load capacity.

Matching tires to boat, utility, and camper trailers

Different trailer types put different demands on tires. A boat trailer carries the weight of the boat, motor, and often water in the bilge; corrosion resistance and flotation are important. A utility trailer may carry heavy, shifting loads like gravel or machinery, so a higher load range helps handle the stress. A camper trailer is often loaded close to its GVWR with water, propane, and gear, so the load index must cover the actual weight.

For all trailer types, the goal is the same: the tire's load rating at the recommended pressure must cover the heaviest wheel position with a margin. If you are close to the limit, choose a tire with a higher load index or a higher load range. Always use tires designed for trailer service. Browse trailer tire recommendations and check the differences between ST and LT tires.

• Boat trailers: Weigh the boat and trailer together, and add a margin for water absorbed by the hull. Use a load range that matches the axle rating, and inspect for corrosion on the rims and tires.
• Utility trailers: Check the payload rating on the label and add the trailer's own weight. If loads can shift, step up one load range for extra safety.
• Camper trailers: Include fresh water, propane, and all cargo when calculating the loaded weight. Many larger units use load range D or E tires with a high load index.

Add a safety margin and maintain your tires

Federal standards set a minimum, but a safety margin is smart. Loads can shift, tires heat up on long hauls, and under-inflation reduces load capacity. Choose tires rated well above the calculated minimum if your budget allows. A margin also helps absorb the extra load that occurs when braking or cornering, especially on tandem-axle trailers where one wheel may momentarily carry more than its share.

Check the tire pressure before every trip, using the pressure recommended by the trailer manufacturer or the tire sidewall for the load you are carrying. Under-inflated tires run hotter and are more likely to fail. Trailer tires also age faster than they wear, so inspect them for cracks, bulges, and sidewall damage. Replace them after six to ten years regardless of tread depth, as rubber degrades over time. Learn the basics of tire pressure and understand tire aging.

What to pick for your use

Questions

What is the difference between load index and load range?

Load index is a numeric code that maps to a specific maximum load at a given inflation pressure. Load range is a letter (C, D, E, F) that indicates the ply rating and maximum inflation pressure. They work together: a higher load range allows higher pressure, which increases load capacity, while a higher load index means more weight at that pressure.

How do I find the GVWR of my trailer?

The GVWR is printed on the certification label attached to the trailer, usually near the tongue or on the frame. It lists the GVWR, GAWR for each axle, and the recommended tire and inflation information. If the label is missing, weigh the trailer at a certified scale to estimate loads.

Can I use passenger car tires on my trailer?

It is not recommended. Passenger car tires are not designed for the constant load and heat of trailer service. ST (special trailer) tires are built to FMVSS 119 and have a different load formula. Using P-metric tires on a trailer can lead to overloading and blowouts.

What happens if my trailer tires are underrated?

Underrated tires will run hotter, increase the risk of blowouts, and can cause handling problems. Federal regulations require the sum of tire load ratings on each axle to be at least the GAWR. If you exceed the load rating, the tire can fail.

How often should I replace trailer tires?

Trailer tires should be replaced every six to ten years, regardless of tread depth, because rubber degrades from heat and ozone exposure. Inspect them for cracks, bulges, and sidewall damage before each trip, and replace them if you see any of these signs.