The Load Range Letter on light truck tires indicates their ply rating.

There is a common misconception that there is a specific equivalence between a tire's Load Range (or ply rating) and its inflation pressure at which it achieves its maximum load. In reality, tires of the same Load Index may require dramatically different pressures to achieve their published load ratings as a result of construction differences.

This can be seen by consulting the standards published every year by the industry standards group, the Tire and Rim Association. As an example, their 2010 Year Book shows that Load Range "C" tires may require 35 psi (240 kPa) or 55 psi (380 kPa), Load Range "D" tires may require 50 psi (340 kPa) or 65 psi (450 kPa), and Load Range "E" tires may require 65 psi (450 kPa) or 80 psi (550 kPa) to achieve their Load Index and Maximum Loads. This is also the case in the tables published by major tire makers, who do follow these TRA standards and have multiple inflation pressures for the same Load Range in the tires they sell.

It is essential to consult the guides like those just mentioned when making tire substitutions, and to read exactly what is imprinted on the sidewalls of tires. A different size of tire with the same Load Range may require a higher inflation pressure, and may fail in use if under-inflated.