Tires for light trucks have other markings besides those found on the sidewalls of passenger tires.
The "LT" indicates the tire is for light truck. These tires are manufactured especially for trucks and the expected loads they will carry. They perform differently than passenger tires and are less safe on a car even if they are the same size.
It used to be that ply number indicated the number of layers of rubber-coated fabric in the tire. In general, the greater the number of plies, the more weight a tire can support. Tire manufacturers also must indicate the materials in the tire, which include steel, nylon, polyester, and others. Today's load range/ply actually indicates the equivalent strength that early ply tires had and do not represent the actual number of ply layers found inside the tire. In fact most radial passenger tires today only 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 body plies, or one steel ply.