Making sense of the information contained on tire sidewalls.
By Bill Hendrix, F761S
The Federal Department of Transportation requires that certain standardized information be molded into the sidewall of all passenger car and truck tires. Reading this information is a bit of a challenge, as some of the numbers and letters are self-explanatory but others are in code for conservation of space. The information on passenger car tires is slightly different from that on truck tires, so let’s look first at the passenger car tire sidewall.
Starting with the outside ring of larger letters, the manufacturer’s name is very prominent. Going clockwise, the next group of letters and numbers is called the Uniform Tire Quality Grading System (UTQGS). The number after “Treadwear” indicates how well the tire wears. The number does not predict the number of miles you might expect but is useful for comparison. For example: A tire marked 400 should last twice as long as a 200. The “Traction” letter is an indication of the tire’s ability to stop on wet pavement; AA is best, followed by A, B, and finally C. The “Temperature” letter indicates the tire’s resistance to heat buildup; A is best, B is next, and C is last.
Next is the name of the tire followed by the letter “P” for passenger, then the nominal width in millimeters, followed by a slash and then the ratio of height to width (aspect ratio). Aspect ratios of 70 or less indicate a short sidewall for better steering response, and numbers above 70 normally would give a better ride quality. Next is an “R” for radial tires, and last in that group is the rim diameter in inches.
The next group of letters and numbers designates the load index and speed symbol; neither is required by law, so you may have to refer to the tire manufacturer’s technical data sheet for specific values. The speed symbol letter starts at “Q” for 99 mph, graduating to “U” at 124 mph, then “H” for 130 mph, “V” for 149 mph, “W” for 168 mph, and “Y” for 186 mph (however, some tire manufacturers use “ZR” to indicate anything more than 149 mph).
“M+S” or “M/S” relates that the tire has some mud or snow capability.
The inside ring is pretty self-explanatory “” tire ply composition will tell how many real plies are in the sidewall and how many real plies are on the tread. Materials used for the plies may be steel, polyester, nylon, etc.
Next are maximum load rating and maximum inflation pressure, probably the most relevant of the markings. The load rating tells how much weight the tire can support at maximum inflation. For tires made for both the U.S. market and countries on the metric system, the maximum load may be expressed in kilograms (kg) and pounds (lbs), and the maximum inflation may be expressed in kilopascals (kPa) and pounds per square inch (psi).
Naturally, if the tire is underinflated, the load capacity is reduced. The Recreation Vehicle Safety Education Foundation (www.rvsafety.org; 321-453-7673) has information available on the importance of weighing each wheel of the vehicle and having the appropriately rated tires, inflated at the corresponding pressure for safe driving.
The U.S. Department of Transportation Identification Number, starting with “DOT,” deserves some explanation. Prior to December 2003, this group was often placed on the opposite side of the tire but is now mandated to be on the outside where you can readily see it. The two digits after DOT indicate a code for the plant where the tire was made. The last group of numbers discloses the week and year of manufacture.
Starting in January 2000, the date code is mandated to be four numbers. The first two numbers designate the week, and the last two numbers indicate the year (WWYY). For example: “0903” would mean the tire was made the ninth week of 2003. Prior to January 2000, the date code could be three numbers, three numbers followed by an arrow, or four numbers. The three-number code would be WWY with Y being the last digit of the year. For example: “069” equals the sixth week of year 1999, or 1989, or 1979, etc. Three numbers with an arrow is also WWY but the arrow, possibly used exclusively by Michelin, designates the 1990 decade. For example: “069
The other letters and numbers between the factory code and the date code are marketing codes that are used at the discretion of the tire manufacturer and could be useful in the case of a recall.
Truck tires require additional markings. “LT” is the abbreviation for light truck. After the diameter number is the load range letter previously referred to as a ply rating, but that is now an indication of the range of load capacity and inflation. The letter “A” replaces 2-ply, the letter “B” replaces 4-ply, etc. So if the marking reads “LRE,” the “LR” is load range and the “E” would be the approximation of the old 10-ply construction. This is really not very useful, since the maximum load number is the one that really counts. Also disclosed are the limits of load and inflation when used as a single and when used as a dual.
Wow, that’s a lot of information embedded on every highway tire made, and most of the information is critical to safety. Check your tire’s weight and inflation, and inspect them frequently. Tires contain compounds of lubricants and antioxidants that activate with use. The compounds are there to preserve the tire, make the tire run cooler, and protect it against damaging ultraviolet rays from the sun. If you use a tire dressing or cleaner, it should not contain any alcohol, silicon, or petroleum products. Protect the tires when in storage from direct sunshine.
When it comes to motorhome tires, a fair rule of thumb for replacement on tire age, from the date of manufacture, might be: five years are usually okay, six years possibly, seven years and you may be at risk. It’s different for passenger car tires, because we usually subject them to more miles and wear them out before they have a chance to age.
A good information piece about tires is available for download at www.nhtsa.dot.gov, Tire Safety Brochure (DOT HS 809 361 dated October 2001).
Take care of your tires like your life depends on it “” because it does.