The load range/ply rating branded on a tire’s sidewall identifies how much load the tire is designed to carry at its industry specified pressure. Today’s load range/ply ratings do not count the actual number of body ply layers used to make up the tire’s internal structure, but indicate an equivalent strength compared to early bias ply tires. Most radial passenger tires have one or two body plies, and light truck tires, even those with heavy-duty ratings (10-, 12- or 14-ply rated), actually have only two or three fabric plies, or one steel body ply. Passenger tires feature named load ranges while light truck tires use load ranges that ascend in alphabetical order (letters further along in the alphabet identify stronger tires that can withstand higher inflation pressures and carry heavier loads).
Before load ranges were adopted, ply ratings and/or the actual number of casing plies were used to identify the relative strength with higher numeric ratings or plies identifying tires featuring stronger, heavier duty constructions. In all cases, when changing tire sizes or converting from one type of size to another, it is important to confirm that the Load Index in the tire’s service description of the new tire is equal to or greater than the Load Index of the original tire and/or that the new tire’s rated load capacity is sufficient to carry the vehicle’s gross axle weight ratings.
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