LT vs P-metric tires: load range and ply ratings

The letters before or after a tire size tell you which rating system the tire belongs to. U.S. tire makers recognize seven systems for passenger and light-truck tires. A size with no prefix at all is a Euro-metric passenger tire, which is now the most common marking on new cars and crossovers. For the rest of the size code, see how to read a tire size.

Light-truck tires also fall under a stricter definition in federal law: an LT tire is one its maker designates for use mainly on light trucks and multipurpose vehicles. Very deep-tread LT tires (18/32 inch or more, typical of some mud-terrain tires) and ST trailer tires are certified to a separate standard, FMVSS No. 119.

A standard-load P-metric tire reaches its maximum rated load at 35 psi (36 psi for Euro-metric sizes). An XL or “reinforced” tire is built to take more air: it reaches its maximum load at 41 psi (42 psi Euro-metric). Below 35 psi an XL tire carries exactly what an SL tire of the same size carries. Its extra capacity only exists at the extra pressure.

The number molded on the sidewall as “max press” is a different figure. It is the most air the tire can safely hold, commonly 44 or 51 psi on standard-load tires and 50 psi on XL tires. The room between 35 and 44 psi lets automakers specify higher pressures for handling or efficiency. It does not add rated load, and it is never the pressure you should run unless your placard says so.

Federal rules require every LT tire to carry its load range letter. Each letter matches an old ply rating and a pressure at which the tire reaches its maximum load. Most LT tires sold for pickups and SUVs are C, D or E. Tires for vehicles of 10,000 lb GVWR or less typically stop at Load Range E, while F is growing on heavier 3/4- and 1-ton trucks and large aftermarket sizes.

A tire doesn’t carry weight with rubber. It carries it with air. That is why every load rating is tied to a pressure, and why the industry publishes load and inflation tables (maintained by the Tire and Rim Association in the U.S. and ETRTO in Europe) that list what each size carries at each pressure.

Here is one LT size across its pressure range, from the Tire and Rim Association figures that Toyo reproduces in its dealer guide. The same tire carries 69% more at 80 psi than at 35 psi, which is the whole point of a high load range.

Compare that with a passenger tire of the same family. A P265/75R16 rated at load index 114 carries 2,601 lb at 35 psi. The LT265/75R16 carries only 1,910 lb at that same 35 psi. The LT tire pulls ahead only above about 50 psi, where it reaches 2,470 lb in Load Range C, 3,000 lb in D and 3,415 lb in E.

When a passenger tire is fitted to a multipurpose vehicle, truck, bus or trailer, federal safety standard FMVSS No. 110 requires its load rating to be divided by 1.10 before checking that the tires can carry the axle. In practice the tire keeps about 91% of the load printed on its sidewall. The rule applies to Euro-metric passenger sizes as well as P-metric, and Toyo describes it as an allowance for the harder use passenger-type tires can see on light trucks. LT tires are not derated.

Load index 112, for example, means 2,469 lb. On a car, that is what the tire is rated for. On an SUV or pickup, the same passenger tire counts for 2,469 ÷ 1.10, or about 2,245 lb. An LT tire with the same index keeps the full figure.

The same standard adds a reserve: the tire’s normal load on the vehicle may not exceed 94% of its rating at the placard pressure. Your truck’s factory tires were chosen with both rules applied, so a same-size replacement needs no extra math. The arithmetic only matters when you change tire type or size.

Owners of half-ton pickups and body-on-frame SUVs often move to LT tires for towing, hauling or off-road use. Tire makers describe LT tires as built for more severe service, with stiffer sidewalls and heavier-duty materials. That is a reasonable choice, as long as you account for what changes.

Toyo’s dealer guide works through a 2006 Ford F-150 that came on P255/70R17 110S tires with a 35 psi placard pressure. That tire carries 2,337 lb at 35 psi, which becomes 2,125 lb after the 1.10 truck derating. The owner wants LT265/70R17 Load Range E tires.

At the placard’s 35 psi, the LT tire carries only 1,890 lb, less than the tire it replaces. At 40 psi it reaches 2,075 lb, still short. It takes 45 psi, where it carries 2,255 lb, to match the original. Left at the placard pressure, the “heavier-duty” tire would be overloaded. Toyo’s thermal images show an under-inflated LT tire running noticeably hotter under the same load, and heat is what breaks tires down.

Expect a firmer ride from the higher pressures and stiffer construction, plus extra weight that can change how the truck steers and stops. The U.S. Tire Manufacturers Association also warns against mixing passenger and LT tires on the same vehicle, so change all four. Better tires don’t raise your truck’s payload or towing rating either: an axle’s rating is set by its weakest part, and Michelin notes that a higher tire rating doesn’t raise the vehicle’s load limit.

Many LT and C-type tires carry two load indexes, written like 121/118. The first is for the tire used alone at the end of an axle. The second is for each tire when two are mounted side by side on a dual-rear-wheel truck. For 121/118 that means about 3,195 lb single and 2,910 lb per tire in dual use.

The dual figure is about 9% lower because the two tires don’t share the load evenly. On a crowned road the inner tire carries more, and inner duals also run hotter. On a single-rear-wheel pickup, only the first number applies.

One recent wrinkle: a 2022 Tire and Rim Association update raised the indexes of some 17-inch LT sizes without changing the size or load range. LT265/70R17 Load Range E moved from 121/118 to 123/120 at the same 80 psi. If your door placard lists an index above 121, a tire built to the older standard may not meet it, so compare the index, not just the load range.

ST (special trailer) tires are built for towed trailers in highway service, and federal rules allow them only on trailers. Never fit them to a car, pickup or SUV. ST tires also carry more than LT tires of the same size and load range: an ST225/75R15 Load Range D is rated 2,540 lb at 65 psi, while the LT225/75R15 Load Range D is rated 2,205 lb at the same pressure, about 15% less.

Older ST tires without a speed symbol are rated for 65 mph. Goodyear’s guidance for those tires is to add 10 psi for speeds of 66 to 75 mph, without counting on any extra load capacity, and only if the wheel can take the pressure. If you put LT tires on a trailer instead, Goodyear says the pressure or size must be raised to meet the trailer’s load, because LT tires carry less. And replace ST tires with ST tires.