by 
The Curious Bag
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Ply rating is an older method to determine the load carrying capacity of truck and trailer tires. Although ply rating is not a formalized tire rating system, it can be used to describe the load capacity of modern truck tires.
Function
Plies are layers of fabric or cord that give a tire shape and strength. More plies equal a stronger tire that can carry a heavier load.
Identification
Truck and trailer tires have their load capacity designated by a letter ranging from A to N, skipping a couple of letters along the way. Equivalent ply ratings step by two: Load range A equals two plies, B equals four plies, etc., up to 24 plies for load range N.
Considerations
The actual load carrying capacity of a tire is determined by the load rating and the tire size. Tire stores and manufacturers can provide the load charts. Tires with the same size and load rating letter can carry the same load.
Types
Light truck tires are usually load range C, D and E, equating to 6-, 8- and 10-ply. Semi-trucks are fitted with G-14 ply- or H-16 ply-rated tires. The 20- and 22-ply L- and M-rated tires are used on the front axles of dump and cement trucks to support their heavy loads.
Fun Fact
A modern tire will not actually have the number of plies indicated by its load range. Current tire manufacturing techniques and materials allow stronger tires without piling on the plies.
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