Indy Here is a chart of a tires Load range or ply rating. As you can see a D would be a 8 ply tire, and a E would be a 10 ply tire. The number of ply's effects how much weight a tire can support. The load of a tire is determined by the manufacture and it's not always true but in general a tire with more plys can handle a heaver load. My truck (2500HD) came with a E rated tire and a towing capacity 10100 lbs When I changed to a D rated tire it was not able to handle the same load as an E rated tire and lowered the towing capacity of the truck. Now I am back to a E rated tire and the towing capacity is back to 10100 lbs.
A truck's towing capacity is determined by a formula which takes into consideration a number of variables. Including, but not limited to, chassis structure (frame strength, wheel base, track, suspension, wheels, tires etc.), engine, transmission, brakes, axle strength, axle ratio and so on. The reasons behind these maximum ratings are to avoid excessive stress on the vehicle in question, and SAFETY
That is why on heavy duty trucks that a made for hauling heavy trailers, and on semi's they run dual back tires. Two tires can handle a heaver load then a single tire.
So the rating of a tire has nothing to do with the tread or how long a tire will last before the tread wears out, the tread is the outer most layer of the tire.
Load rangeThe Load Range Letter on light truck tires indicates their ply rating.[7]
Load Range Ply Rating
A 2
B 4
C 6
D 8
E 10
F 12
G 14
H 16
J 18
L 20
M 22
N 24
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.[8] In reality, tires of the same Load Index may require dramatically different pressures to achieve their published load ratings.
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.[9] 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. [10]
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.
