Originally Posted by checkmate99
Since 2000, the NHTSA has required that all passenger car tires, as well as, all pneumatic mfg tires, have the load range, max cold tire inflation pressure, speed rating etc...stamped on the sidewall of all tires sold in the United States. I looked it up!
Load range is not applicable to passenger tires.
The current description method for speed ratings has been used since 1971.
Speed letters have not and are still not mandatory for ST tires. The current adaption of service descriptions including speed letters for ST tires has been forced on those tires being imported. To pass customs without having to pay an increased tariff most ST tires must now have speed letters.
The official load capacity medium for ST and LT tires is the load range letters. They are also official for medium to heavy duty truck tires.
Passenger tires load capacity is presented on their sidewalls as a service description. Something like 100Q where the 100 will lead you to a chart depicting its load capacity and the "Q" is its speed restriction. Passenger tires also have a grading system called "Uniform Tire Quality Grade (UTQG) Standards".
The DOT allows the use of service descriptions on ST tires although they are not the official method used to determine their load capacity.