Tire Types: Tire Load Capacities & Pressure for Euro-metric, P-metric Tires

# Tire Types and Load Capacity

#### Swapping P-metric for LT-metric requires key load considerations and calculations

The most important consideration when selecting replacement tires for a van, pickup truck or SUV is to maintain sufficient load carrying capacity. A vehicle and its load are completely supported by the inflated tires. If the installed tire’s load rating is insufficient for the vehicle, even a sound tire may become overheated, creating a real risk of tire failure.

The maximum load and maximum inflation ratings can be found on the sidewall of all tires. These figures specify the maximum safe load that can be placed on a tire and the maximum allowable tire pressure.

A tire’s internal construction determines its ability to withstand pressure. Stronger construction allows the use of higher inflation pressures, and, for a given size, increases the load a tire can carry. It is important to never exceed a tire’s maximum load rating, or the maximum vehicle load limit shown on the vehicle tire placard (whichever is less); regardless of what a customer may request.

Three tire-sizing systems are used on the OE tires fitted to today’s light trucks: Euro-metric; P-metric (passenger metric); and LT-metric (light truck metric).

Euro-metric tires (such as 235/70­R16) originated in Europe in the 1960s and originally were intended to use metric measurements to indicate a tire’s section width. As more profiles and load capacities were demanded by automobile manufacturers, tire companies responded with new sizes.

P-metric tires (such as P235/70R16) were introduced by U.S. tire manufacturers in the 1970s and are designed for use on vehicles principally intended to carry passengers. The load capacities of P-metric tires are the result of an engineering formula that includes both the physical dimensions of the tire and its maximum inflation pressure.

Euro-metric and P-metric tires of the same numerical size (e.g., P205/55R16 and 205/55R16) have equivalent dimensions and vary only slightly in load capacity. If Euro-metric and P-metric tires of the same numeric size also have the same performance category and the same speed rating, they are interchangeable and functionally identical if used in axle pairs or sets of four. For the rest of this article, we’ll simplify things by referring to Euro-metric and P-metric tires as “P-metric.”

P-metric tires are rated for either Standard Load or Extra Load capacities. Most Standard Load tires have a maximum inflation pressure of 35 psi and their load carrying capability reaches a peak at that pressure. Raising the air pressure does not increase the load capacity.

Some Standard Load tires are marked with a maximum inflation pressure of 44 psi, but are still rated for their maximum load capacity at 35 psi. The additional inflation pressure may allow the tire to meet special performance requirements, but it does not increase the tire’s load carrying capacity.

Extra Load-rated tires are designed for a maximum inflation pressure of 41 psi and their maximum load capacity is reached at that inflation. Usually Extra Load tires have an “Extra Load” marking and may be identified by an “XL” or “RF” (reinforced) in the size designation (e.g., P235/75R15 XL). There are a few vehicles that require XL tires to support the weight of the vehicle.

LT-metric tires were specifically developed for use on light trucks and have a number of characteristics that differentiate them from P-metric tires. An LT tire is built more robustly than a P-metric tire, primarily to handle heavy loads under adverse conditions. Heavier body plies and larger bead bundles allow LT tires to be inflated to higher pressures, increasing the tire’s load capacity and allowing them to carry a heavy load continuously (not just occasionally) without overheating and failing.

All load range “C” tires (the old “6-ply” rating) can be inflated to 50 psi, “D” (8-ply) to 65 psi and “E” (10-ply) to 80 psi. LT tires are designed to minimize internal heat and to endure elevated internal heat levels for long periods. Typically, LT tires are made of higher quality materials than P-metric tires, and their construction is much more robust, with heavier sidewalls and denser, higher quality steel belts.

LT tires also are designed to endure greater physical abuse than P-metric tires, and some LTs are similar to the heavy-duty tires fitted to 18-wheelers and buses, using all-steel construction. LTs can handle sidewall abrasions better than P-metrics and are better suited to off-road and construction site use. Usually, LT tires have deeper tread depths than their P-metric counterparts.

The trade-offs for these strengths include significantly higher weight, a stiffer ride, increased rolling resistance and reduced fuel efficiency. The differences in load capacity and required inflation pressure prohibit mixing LT-metric tires with P-metric tires. It also creates problems when trying to replace P-metric tires with dimensionally equivalent LT-metric tires, and vice versa.

P vs. LT
Although originally developed for cars, most vans, pickups and SUVs are used primarily to carry passengers and P-metric tires are typically lighter, with lower rolling resistance and less aggressive tread designs than typical LT-metric tires.

The combination of better ride, greater fuel efficiency and less noise has resulted in P-metric tires being used extensively for light truck applications. However, vans, pickup trucks and SUVs typically have a higher center of gravity and greater probability of being overloaded than passenger cars.

In order to accommodate this, vehicle engineers are required by Federal Motor Vehicle Safety Standards to specify P-metric sized tires rated to carry 10% more weight than would be required if they were used on a passenger car. This is the equivalent of dividing the tire’s sidewall branded load capacity by 1.10. For example, a P265/75R16 114S tire has a maximum load rating of 2,601 lbs. at 35 psi. However, when this tire is installed on a light truck, the load rating is reduced by a factor of 1.10 to 2,365 lbs. at 35 psi.

If you replace a P-metric OE tire with an LT-metric tire, it must be of a size and load range that will offer equal or higher load carrying capacity compared to the OE tire on the vehicle. The ability of any tire to support a specific load is solely based on the inflation pressure within the tire, and the maximum load and inflation capability of the tire. Even tires with the most robust design and construction will fail in service when underinflated.

Size-for-size, compared to P-metric tires, LT tires require higher air pressures to carry equivalent loads.

Failure to adjust air pressures to achieve the vehicle’s load requirement will result in tire fatigue and eventual tire failure due to excessive heat build-up. Because of the higher inflation required by LT tires, they often are not appropriate replacements for OE P-metric tires due to ride harshness caused by higher inflation pressure.

As an example, consider an OE P265/75R16 114S with a maximum load of 2,601 lbs. at 35 psi. If a LT265/­75R16 Load Range C is to be used as a replacement, to carry an equivalent load the LT tire must be inflated to 50 psi. Even an LT265/75R16 Load Range D or E tire must be inflated to 50 psi to carry the load the P-metric tire carries at 35 psi.

The Tire and Rim Association Yearbook can be used to find the load of the original tire according to the inflation pressure shown on the vehicle’s tire information placard. You can then refer to the “Light Truck” load and inflation tables and apply the inflation pressure that corresponds to the required load of the P-metric tire.

On the other hand, if you replace an OE LT tire with a P-metric tire, check the load requirements carefully.

Many times, the P-metric tire does not offer enough load capacity. Also remember that you must reduce the P-metric loads by a factor of 1.10 when replacing OE LT tires. Always select a tire that offers a greater or reserve load capacity, which will help the vehicle handle and respond to higher-stress emergency situations. Here, too, the Yearbook can help determine the appropriate tire and inflation pressure.

Another consideration when replacing OE LT tires with P-metric tires is that LT tires feature heavier construction in the sidewall, shoulder and tread areas and are better suited to deal with off-road conditions. A P-metric replacement tire should be considered only if the vehicle will not be operated off paved roadways.

Where LT-metric duals are fitted to a light truck, replacing the dual tires with P-metric tires generally is not recommended.

The most important question is probably, “What did the vehicle come with from the factory?” LT tires have a much higher spring rate than P-metrics, and changing the tire’s spring rate can produce some unintended, and perhaps dangerous, changes in the handling of the vehicle. Springs, shocks and sway bars were all designed by the vehicle manufacturer with the original type of tire and its properties in mind.

Caution and common sense dictate that it’s probably best to stay with the type of tire the vehicle manufacturer specified, rather than re-engineer the vehicle.

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