The alpha-numeric tire code consists of a string of letters and numbers, as follows:
An optional letter (or letters) indicating the intended use or vehicle class for the tire:
P: Passenger Car
LT: Light Truck
ST: Special Trailer
T: Temporary (restricted usage for "Space-Saver" spare wheels)
Use of the letter P indicates that the tire is engineered to TRA standards and absence of a letter indicates that the tire is engineered to ETRTO standards. In practice, the standards of the two organizations have evolved together and are virtually interchangeable.
3 digit number: The "nominal section width" of the tire in millimeters; the widest point from both outer edges.
/: Slash character for character separation.
2 or 3 digit number: The "aspect ratio" of the sidewall height to the total width of the tire, as a percentage.
An optional letter indicating construction of the fabric carcass of the tire:
B: bias belt (where the sidewalls are the same material as the tread, leading to a rigid ride)
2 digit number: Diameter in inches of the wheel that the tires are designed to fit. There is the rare exception metric diameter tires, such as the use of the 390 size, which in this case would indicate a wheel of 390 mm in diameter. Few tires are made to this size presently.