Tire Load Index for Towing: How to Calculate Safely

Why Load Index Matters for Towing

When you tow, the weight of the trailer and its cargo transfers to the vehicle's rear axle and directly to the trailer's own axles. Each tire must support its share of that load without exceeding its maximum load rating. The load index, a number molded into the sidewall, is a code that corresponds to a specific maximum load capacity at a given inflation pressure. The actual capacity in pounds is found in a load index chart, which you can review in our tire load index rating article.

Federal safety standards require that the tires on any axle are not overloaded. For vehicles with a gross vehicle weight rating (GVWR) of 10,000 pounds or less, the sum of the maximum load ratings of the tires fitted to an axle must be at least equal to the axle's gross axle weight rating (GAWR) as shown on the certification label. For heavier vehicles, the same rule applies, but the exact wording is structured differently. Either way, the principle is the same: never fit tires that cannot handle the heaviest load the axle can legally carry.

How to Calculate the Required Load Rating

Start by finding the GAWR for each axle on the vehicle and on the trailer. This number appears on the certification label, usually on the driver's door jamb for vehicles and on the tongue or frame for trailers. The regulation defines GAWR as the gross axle weight rating, and it is the maximum weight that the axle is designed to carry.

For a single-axle setup with two tires, divide the GAWR by 2 to get the minimum load rating each tire must have when the tires are identical. For dual wheels on an axle, divide by 4. The sum of the maximum load ratings of all tires on that axle must be no less than the GAWR, but if you spread the load evenly across identical tires, each tire needs at least one-half (or one-quarter) of the GAWR. For a complete list of load indices and their capacities, see our tire load capacity chart.

Once you have this number, look up a load index that provides at least that capacity at the pressure you will use. Do not use the maximum pressure printed on the tire sidewall as your guide; use the vehicle manufacturer's recommended cold inflation pressure, which is listed on the placard. The load rating at that pressure must meet the requirement.

Adjusting for Passenger Car Tires

Federal rules include a special adjustment when a passenger car tire (marked with a P, such as P225/60R16) is installed on a multipurpose passenger vehicle, truck, bus, or trailer. In that case, the tire's load rating must be reduced by dividing by 1.10 before you compare it to the axle weight. This means a P-metric tire rated for 1,700 pounds is treated as only about 1,545 pounds on a truck or trailer. Failing to make this adjustment is a common mistake that can lead to an under-rated tire.

Light truck (LT) tires do not require this reduction because they are designed for heavy loads from the start. Many trailers come with special trailer (ST) tires, which are built for the high sidewall flex and heavy loads of trailer service. Check the sidewall to see whether your tires are P-metric, LT-metric, or ST, and apply the 1.10 factor only to P-metric tires.

Worked Example: Calculating for a Trailer

Suppose you have a tandem-axle trailer with a GVWR of 6,000 pounds. Each axle has a GAWR of 3,000 pounds, and each axle carries two tires. For each axle, the sum of the maximum load ratings of the two tires must be at least 3,000 pounds, so each tire must be rated for at least 1,500 pounds.

If you are considering P-metric tires for this trailer, divide the required capacity by 1.10 first. The sum after division must still be at least 3,000 pounds, which means each P-metric tire must have a load rating of about 1,650 pounds before the adjustment. An LT or ST tire with a load index that provides at least 1,500 pounds at the trailer's recommended pressure works without the reduction.

If the trailer has a single axle with a GAWR of 3,500 pounds and two tires, each tire needs at least 1,750 pounds. Use the load index chart to find the lowest index that meets that number, but also ensure the tire's intended service (trailer use) matches the vehicle. For a boat trailer, for example, corrosion-resistant wheels and tires designed for trailer service are worth prioritizing.

Trailer Tires and Load Range

Trailers in highway service may use ST (special trailer) tires, which are covered by Federal Motor Vehicle Safety Standard No. 119. These tires have stiffer sidewalls and higher load capacities at lower pressures than comparable passenger tires. If you are replacing tires on a trailer, see our best trailer tires guide and the article on trailer tire selection for more details.

Load range (such as C, D, E) is another way to express load capacity alongside the load index. A higher load range corresponds to a higher maximum inflation pressure and, in turn, a higher maximum load rating. When towing heavy loads, many pickup owners choose LT tires with load range E or higher. For trailers, ST tires with load range D or E are common for weight capacities above 5,000 pounds. The load index still tells you the actual capacity, so always cross-check the two.

Common Mistakes to Avoid

One frequent error is relying on the sidewall's maximum pressure as a guide. The vehicle placard lists the recommended pressure, and the load rating at that pressure is what matters. Over-inflating to the sidewall maximum to gain load capacity can cause uneven wear and a harsh ride.

Another mistake is using tires with a lower load index than the original equipment when replacing tires. Even if the new tire is the same size, a different load index may be necessary for towing. Also avoid mixing tires with different load indices on the same axle. Finally, remember that the 1.10 reduction for P-metric tires applies to any truck, bus, or trailer, not just heavy-duty vehicles. Ignoring this adjustment is a leading cause of overload failure.

What to pick for your use

Questions

What happens if my tires are underrated for towing?

If the sum of the tires' maximum load ratings on an axle is less than the GAWR, the tires can overheat and fail, leading to a blowout or loss of control. The vehicle's handling and braking also degrade, especially at highway speeds.

Can I use the tire pressure printed on the sidewall for load calculations?

No. The sidewall shows the maximum permissible inflation pressure, not the recommended pressure. Use the vehicle manufacturer's recommended cold inflation pressure from the placard, and check the load rating at that pressure in a load index chart.

Do I need to inflate tires to maximum pressure when towing?

Only if the vehicle manufacturer specifies a higher pressure for towing. Many vehicles list a separate pressure for heavy loads. Always follow the placard or owner's manual, never just the sidewall maximum, to avoid uneven wear.

Is it okay to use passenger car tires on a trailer?

Federal regulations allow it only if the load rating is reduced by dividing by 1.10. Most trailer tires are ST or LT types designed for the high sidewall stress and heavy loads of trailer service. P-metric tires often lack the structural reinforcement needed.

Where can I find the GAWR of my trailer?

The GAWR is on the certification label attached to the trailer frame or tongue. It may also be listed in the owner's manual. If you cannot find it, contact the trailer manufacturer or weigh the trailer on a scale to determine the actual axle load.