Almost every tire and certainly any you should be looking at, new or used, will have a series of numbers and letters on its sidewall that can tell you just about everything you need to know about that tire. These numbers, usually with a letter or letters in front, describe the its size, type, and construction.
The tires you will most likely be looking at are P-metric and LT. A tire with P in front of the series of numbers on the sidewall means that it’s a P-metric designed primarily for passenger vehicles that may include cars, SUVs, and light trucks (usually 1/2-ton). An LT (for instance, LT315/70R17) in front of the numbers tells you this is a Light Truck Metric meant for medium and heavy-duty (3/4 and 1-ton pickups and SUVs) trucks, and it’s designed to handle heavy loads. A tire with LT after the size numbers is for use on 1/2, 3/4, and 1-ton vehicles that haul heavy cargo or tow trailers, are flotation tires used for sand, or are extra wide tires meant for or use on 16.5-inch wide wheels.
The numbers following or preceding the P or LT can tell you the size of the tire. It’s not simple, but if you need to convert a Metric tire to a Society of Automotive Engineers (SAE) tire size, you can use this formula: ([Tire Width x Aspect Ratio x 2] / 2550 + Wheel Diameter) x (Tire Width / 25.4) – (Wheel Diameter). In other words, a LT315/70R17 is 34.4x12.4R17. The closest equivalent would be a 35/12.50R17LT and that tire’s size is somewhat of a no-brainer. It’s 35 inches tall, and the section width is 12 1/2 inches wide.
If the tire is an LT315/70R17, the letter R after the aspect ratio number tells you that the tire is of radial construction. The letter D signifies that the tire is a bias ply (crossing each other on a diagonal from the wheel center). The number of plies (commonly 4 to 6, but in some military and commercial tires, can often be 10 or 12) is usually, but not always, stamped into the tire’s sidewall.
The maximum load capacity is also molded into a tire’s sidewall. This load capacity is a function of the tire’s size (size of its air chamber), its construction (how much pressure it can hold), and the air pressure (how much air is inside the chamber).
Born On Date/Country Of Origin
You can also determine the tire’s age and country of origin from the Department of Transportation (DOT) Tire Identification Number (TIN), a set of up to 12 letters and numbers that begins with DOT on the sidewall. Since 2000, the week and year of a tire’s manufacture are the last four digits of the TIN. Prior to 2000, they only used three digits because they assumed tires would not be in service for more than 10 years. Face it though, if you’re looking at tires (new or used) older than 2000 at a swap meet or from some dude on eBay, it’s time to look at different tires.
Country of origin can be more difficult to determine. The manufacturer and plant code are the first two letters (or a number and letter, or two numbers) after DOT. Due to the increased number of manufacturers and plants in recent years, the DOT has instituted a three-number code for some tires. Plant codes can be found on the National Highway Traffic Safety Administration’s (NHTSA) website search page (nhtsa.gov/apps/manufacturer/index.htm). You can enter the manufacturer and the plant code into NHTSA’s searchable database for the information.