A tire's ability to carry a heavy load primarily depends on its air pressure. If a car has an underinflated tire, the car's weight puts pressure on the tire and distorts its round shape, overloading it and impairing the tire's ability to roll smoothly. When the tire is inflated, the densely packed air inside the tire pushes back against the vehicle's weight, maintaining the tire's roundness and allowing it to roll properly without excessive friction or heat buildup.
The more air pressure a tire has, the more weight it can safely support. Truck and trailer tires come in larger sizes than passenger tires and are built with more durable construction. This allows the tires to contain more air and to be inflated to higher air pressure levels, allowing them to support heavier loads. There are two parameters used to measure a tire's ability to carry loads: load capacity, indicated by the Load Index; and maximum load pressure, indicated by the Load Range.
The Load Index, or load rating, is a number located in the service description on a tire's sidewall. This number represents the tire's load capacity, the maximum amount of weight that the tire has been tested and approved to carry.
A load index is useful in comparing the load capacities of different tires; the higher the load index, the more weight that tire can support. However, the load index itself isn't an actual measure of weight. Instead, each numeric value corresponds to a particular amount of weight, representing the heaviest load that the tire can carry. For instance, a tire with a sidewall code of 185/65R15 88H has a load index of 88, which means that the tire can carry loads weighing up to roughly 1,235 lbs (560 kg).
For safety reasons, a new tire should always have the same load index as the tire it replaces or higher. In order to support a vehicle properly, the combined load-carrying capacity of all four tires must be at least the vehicle's weight. For example, a sedan that weighs 2,426 lbs cannot use tires with a maximum load less than 607 lbs.
Of course, load-carrying capacity should not only account for the vehicle's curb weight, but the weight of the driver, passengers, and cargo as well. So for practical purposes, our 2,426 lb sedan might need a set of P175/65R14 82H tires, which can carry 1047 lbs per tire or 4188 lbs total.
Unlike a passenger tire, truck and trailer tires have two load indexes. One indicates the load capacity if the tire is on a regular axle with one wheel, while the other indicates the load capacity if the tire is on a dual rear wheel axle. Typically, the load index is lower for two tires than for one tire. This is a precaution to ensure that if one tire on a dual wheel axle fails, the other tire can still carry the axle's full load.
Today, many trucks and SUVs are equipped with passenger tires, which have a smoother ride quality than light truck tires. However, a light truck also tends to have a higher center of gravity than a passenger car, making it easier to overload the tires. Since passenger tires were designed for use on cars and not trucks, they are only able to support about 91% of their total load capacity when fitted on a light truck or SUV.
For a pickup truck with a gross vehicle weight rating of 6800 lbs, each tire needs to carry at least 1700 lbs. With passenger tires, each tire would need a load index of 102 or higher, which translates to a load capacity of 1874 lbs on a passenger car and about 1705 lbs (91 percent of 1874 lbs) for a light truck. On the other hand, since light truck tires can carry their full maximum load on a light truck, each tire would only need a load index of 99, or a load capacity of 1709 lbs.
A tire's load range refers to its maximum load pressure, the air pressure level at which the tire demonstrated the capability to carry its maximum load. A tire's "maximum load pressure" should not be confused with and is often lower than the "maximum inflation pressure," which is the highest air pressure to which the tire can be safely inflated.
Tires for light trucks, trailers, and large commercial vehicles indicate their load range on the sidewall using a letter from A to N in increasing order, each indicating the standardized air pressure level at which the tire's load-carrying capacity was tested. Most light truck tires have a load range of C, D, or E, corresponding to maximum load pressures of 50 psi, 65 psi, and 80 psi respectively.
Each load range also corresponds to a "ply rating", which is a measure the strength of the tire's internal construction. (Originally, the ply rating counted a tire's ply layers; however, today's tires use fewer, stronger ply layers.) An increased ply rating results in an increased load range, as a tire with a stronger casing can be inflated to a higher pressure to accommodate heavier loads.
Passenger tires do not definie their load range using this scale. Instead, passenger tires are categorized as Standard Load (SL) or Extra Load (XL). Most passenger tires are Standard Load, which means that the tire can support its maximum load when inflated to about 36 psi. Extra Load, on the other hand, means that the tire can carry its maximum load when inflated to an air pressure of roughly 42 psi. An Extra Load tire can be identified by the inclusion of the letters XL or RF on its sidewall, while a Standard Load tire usually does not indicate its load range in the tire code.
Separately from the load index and load range, tires are required to list their maximum load and their maximum air pressure on their sidewall. That's the maximum inflation pressure, not the maximum load pressure: for passenger tires, this will usually be 44 psi or 51 psi, even though the tires are rated to carry their maximum load at a load pressure of 36 psi or 42 psi.
Knowing the maximum load and maximum load pressure of your tires is crucial to avoid overloading your tires. Since a tire's load-carrying capability depends on its air pressure, an underinflated tire will have a lower load capacity than it is rated for. When loading up your car, truck, or trailer, remember to check that the weight is within your total load capacity, and that your tires are inflated to their maximum load pressure.