Load Index and Load Range Explained: P-Metric, XL and LT

Two tires can share a size and still carry very different loads. The difference shows up in the load index, the XL marking on passenger tires and the load range letter on light truck tires. All three come back to one rule: a tire carries its rated load only at the pressure that rating assumes.

What the load index tells you

The load index is the number right after the size, in front of the speed symbol. It is a code for the maximum load one tire can carry under set conditions, and published charts convert it to pounds: 100 equals 1,764 lb, 110 equals 2,337 lb and 121 equals 3,195 lb.

That maximum assumes a specific inflation pressure, and below it the tire carries less. In the TRA passenger table, a P255/70R17 with load index 110 is rated for 2,050 lb at 26 psi and 2,337 lb at 35 psi. The number on the sidewall is a ceiling, not a rating at any pressure.

The index alone also can’t settle a comparison between tires built to different standards. One tire maker’s load-table guide notes that tires with the same load index may need very different pressures to carry it, and that an equal or higher index does not always mean equal or higher capacity at every pressure.

Standard load vs. extra load (XL) passenger tires

Passenger tires come in standard load and extra load versions. Extra load tires are marked XL, Extra Load or Reinforced; if none of those words appear, the tire is standard load. The added capacity comes from running higher pressure, not from carrying more at the same pressure.

The TRA passenger table shows this for a P215/75R15:

• Standard load: 1,742 lb at 35 psi, which is its maximum.
• Extra load: the same 1,742 lb at 35 psi, rising to 1,808 lb at 38 psi and 1,874 lb at 41 psi.

Euro-metric tables follow the same pattern, with standard load topping out at 36 psi and reinforced at 42 psi. If your vehicle came with XL tires, replace them with XL tires of at least the same load index. If you switch between standard load and XL, have the tire shop confirm the pressure that carries at least the original load.

LT load ranges C, D, E and F

Light truck tires use a load range letter instead of an XL marking. Each step up allows a higher maximum pressure and, with it, a higher maximum load. Load ranges are often paired with a ply rating, written as E/10 for example. The ply rating is an index of strength and does not necessarily match the number of layers inside the tire.

The table shows the pressure at which each load range reaches its maximum load in the TRA tables for most LT-metric sizes and for flotation sizes such as 35×12.50R20LT, plus the single-tire maximum for one common LT size.

The load at a given pressure does not change with the letter. At 50 psi, an LT275/70R18 carries 2,680 lb whether it is load range C, D, E or F; a higher letter adds capacity only once you inflate past that point. The exact maximum load and pressure for any tire are molded on its sidewall.

The second number in an LT service description, as in 125/122, is the dual rating. It applies when tires are mounted in pairs on a dual-rear-wheel truck, and it is about 9% lower than the single rating to allow for road crown. For LT275/70R18 in load range E, that works out to 3,640 lb single and 3,305 lb per tire in duals at 80 psi.

Why LT tires need more air to carry the same load

Size for size, an LT tire needs more pressure than a P-metric tire to carry the same weight. One tire maker’s technical bulletin uses a P245/75R16 109 as the example: it carries 2,271 lb at 35 psi. To carry that same load, an LT245/75R16 has to run at 50 psi, whether it is load range C, D or E. The bulletin’s point is that an underinflated LT tire gives you no heavy-duty benefit.

The same bulletin warns that not raising the pressure to suit the load leads to heat build-up, fatigue and eventual tire failure, and its thermal images show an LT tire at 30 psi running considerably hotter than at 50 psi under the same load. It also cautions that the higher pressures can make the ride harsher, one reason LT tires may not suit a vehicle that came with P-metric tires.

Switching between P-metric and LT tires

Federal safety standards require vehicle makers to fit tires whose combined maximum load ratings on each axle are at least the gross axle weight rating (GAWR) on the certification label. When passenger tires go on a truck, a multipurpose vehicle such as an SUV, a bus or a trailer, each tire’s rating is divided by 1.10 for that check. So a P245/75R16 109 rated at 2,271 lb counts as about 2,065 lb on a light truck, and tire makers apply the same reduction when they work out replacements.

If you replace P-metric tires with LT tires, the new tires must carry at least what the originals carried at the placard pressure. One tire maker works through a half-ton pickup example: the original P255/70R17 110S was rated 2,337 lb at 35 psi, or 2,125 lb after the 1.10 reduction. The LT265/70R17 121S load range E replacement needed 45 psi to match it. The same maker says the installer should tell the owner the new pressure and put a decal over the placard showing the new size and pressure.

Going the other way is riskier. The bulletin warns that a P-metric tire often lacks the capacity to replace original LT tires once the 1.10 reduction is applied, and the maker does not recommend P-metric tires as replacements for LT duals.

Who actually needs LT tires

The simplest answer is on the placard. If your truck came with LT tires, replace them with LT tires of at least the same load range and load capacity. Heavy-duty 3/4-ton and 1-ton pickups commonly leave the factory on load range E tires; recent spec sheets list sizes such as LT245/75R17 E and LT275/70R18 E.

Many half-ton pickups and SUVs come with P-metric or Euro-metric tires instead. If you regularly carry heavy loads or tow near your truck’s ratings, ask a tire shop whether LT tires suit your setup, and if you switch, run the pressure the load tables call for. Tires never raise a truck’s GVWR, GAWR or towing rating. Those limits come from the vehicle maker and are printed on the certification label.

Shop the category that matches your placard: light truck tires for LT sizes, and passenger car tires or SUV and crossover tires for P-metric and Euro-metric sizes. Medium- and heavy-duty commercial trucks use a separate class of tires with their own load tables, listed under truck and bus tires. Questions about a listing’s load range can go to Simolary customer service, Monday to Friday, 9 AM to 5 PM Mountain Time, at +1 307-215-1213 or support@simolary.com.