Selecting a replacement tire for a light truck or SUV application sometimes places the customer between a rock and a hard place. It’s up to you to match the tire’s performance requirements with appearance.
The tire’s construction must be able to accommodate the vehicle’s weight and intended use. Blindly choosing a P-metric passenger car tire (even if the tire size meets the criteria) can be a bad idea, if the tire of choice isn’t designed to handle the dynamics of the customer’s vehicle.
According to The Tire Rack, three tire-sizing systems are used on OE tires that are fitted to today’s light trucks: euro-metric, P-metric and LT-metric. Many 1/4-ton and 1/2-ton vehicles use euro-metric and P-metric sized tires, while 3/4-ton and one-ton vehicles use LT-metric sizes.
Euro-metric and P-metric tire sizes were originally designed for cars and station wagons. However, they have also been used for light truck applications because most vans, pickup trucks and SUVs are intended to carry passengers instead of cargo. Additionally, most of the new light trucks being produced today are equipped with euro-metric or P-metric sized tires because they offer lighter weight, lower rolling resistance and less-aggressive tread designs for improved ride comfort, less noise and better fuel economy as opposed to typical heavy-duty tires.
However, The Tire Rack points out that we need to consider the fact that vans, pickup trucks and SUVs typically feature a higher center of gravity and greater likelihood of being overloaded than passenger cars. In order to accommodate this, vehicle engineers are required to specify euro-metric or P-metric sized tires that are rated to carry 10% more weight than would be required for a passenger car application. This is the equivalent of taking the tire’s load capacity and multiplying it by 91%.
For example, a euro-metric or P-metric tire designed to carry 2,000 pounds on a car is restricted to carrying 1,820 pounds when that same tire is used on a van, pickup truck or SUV. This size selection practice provides the vehicle manufacturer with the appropriate tire load capacity.
On the other hand, LT-metric sized tires were specifically developed for use on light trucks, and provide the full tire load capacity that is displayed on the tire sidewall. Because of this, LT-metric sized tires are built very strong and use higher inflation pressures to carry a given load in order to provide the desired safety margin.
While there isn’t a problem in determining an appropriate alternate size when replacing euro-metric or P-metric tires with other euro-metric or P-metric sizes, the differences in load capacity and inflation pressures prohibits mixing the LT-metric tires with euro-metric or P-metric tires or visa versa.
LT vs. P-metric
As noted by Cooper Tire’s John Pecoraro, manager of product marketing, an LT tire is built more robustly than a P-metric tire, to handle heavy loads under adverse conditions. Heavier body plies and larger bead bundles allow LT tires to be inflated to higher pressures, thereby increasing the tire’s load capacity. It is the air inside the tire that carries the load. Usually LT tires have deeper tread depths than their P-metric counterparts. Officially, LT tires should always be replaced with LT tires. P-metric tires are passenger tires that are better suited for on-road use. There are some occasions where an LT tire may be a better replacement application than the OE P-metric, such as when better off-road durability is desired.
By the same token, a P-metric tire may be acceptable for a light truck or SUV application, providing the tire offers sufficient load rating (to handle vehicle weight), and providing the vehicle won’t be driven in an off-road situation. In addition to a concern for load carrying capacity, LT tires feature heavier construction in sidewall, shoulder and tread areas to better withstand rough-terrain exposure.
A P-metric tire should only be considered if the vehicle will only be operated on paved roadways.
As we mentioned earlier, if a set of P-metric tires are to be installed, replacing an original set of LT tires, the P-metric tires should feature a 10% increase in load carrying capacity in order to provide an acceptable margin of safety.
For example, if the vehicle weighs 4,500 pounds, normally each tire should be capable of carrying a minimum of 1,125 pounds. If a P-metric tire size is chosen, the tires should be able to carry at least 1,238 pounds.
Tire load ratings
Tires (and their inflation) are totally responsible for supporting vehicle weight. If the tire’s load rating is insufficient, the tire may become overheated, resulting in the potential for tire failure (through no fault of the tire, if the tire model is not correct for the specific vehicle).
For example, if a passenger car performance tire is selected to replace an OE light truck tire on an SUV, the new tire must meet or exceed the requirement for the specific vehicle weight. Consider the vehicle’s gross weight and divide this by four in order to roughly determine the load capacity for each individual tire. If the vehicle gross weight is 4,500 pounds, each tire should be able to safely support at least 1,125 pounds.
However, you should never select a tire that only meets this minimum weight capability. 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.
The tire’s load rating, or “Max Load” indicates the individual tire’s safe maximum load-carrying capacity, when inflated to its recommended pressure. Regardless of what the customer may initially request, never exceed a tire’s maximum load rating (the limit that is molded into the tire sidewall, or the maximum vehicle load limit shown on the vehicle tire placard, whichever is less).
Load and inflation
The Max Load and Max inflation numbers found on the tire sidewall indicate the maximum load that can safely be carried and the maximum allowable tire pressure. The construction of the tire (belts, bead, carcass, liner) dictates the tire’s ability to withstand pressure. The stronger the reinforcements, the greater pressure the tire can hold.
Most alpha-numeric tires feature a load range of B, which indicates that they are restricted to the load that can be carried at a maximum inflation pressure of 32 psi. C, D or E tires are capable of greater loads. Most load range C, D and E tires are intended for light truck applications.
Tire load-carrying capacity of P-metric tires is rated as either Standard or Extra Load. Standard Load tires are limited by the load that can be supported with a maximum inflation pressure of 35 psi. Extra Load-rated tires are limited to the load that can be carried at a maximum inflation pressure of 41 psi. Generally, a Standard Load tire will not feature a special designation mark, while Extra Load tires will feature an “Extra Load” marking.
Extra load P-metric tires will be branded as “Extra Load” and may be identified by an “XL” (for example: LT245/75R15 XL).
It’s important to note that a Standard Load tire (with a normal inflation pressure recommendation of 35 psi) may be marked with a maximum inflation pressure of 44 psi. This does not indicate an increase of the tire’s load carrying capacity, but indicates the tire’s ability to handle higher inflation pressure in order to accommodate special performance requirements. Generally speaking, load indexes of passenger car tires and light trucks range from 70 to 110.
A speed-rated tire’s sidewall markings will indicate size, followed by the load rating index and the speed rating. For example, a P195/60R15 87S indicates that this tire carries a load rating of 87 and a speed rating of S (this load rating is 1,201 pounds, and the speed rating is 112 mph).
A 285/35ZR19 99Y tire carries a load rating of 99 and a speed rating of Y (this load rating is 1,709 pounds, and the speed rating is 186 mph).