It all began back in 2009, when the National Highway Traffic Safety Administration, set new standards for how tire manufactures mark tires for light trucks and passenger cars. There's a whole lot of information contained in those engraved letters and numbers now. This information includes:
- The type of tire - Start at the tire model name on the outer ring of the tire. Move clockwise around the outside ring of the sidewall. You should run into the letter "P". P stands for "Passenger Tire" "LT" stands for Light Truck.
- The aspect ratio - The next three numbers following "P" gives you the width of the tire in millimeters. Next there will be a slash mark and a second number after the slash. This number is the height of the tire. Put the first number over the second number and you'll have the ratio of width to height. This fraction is the aspect ratio. In the photo above the aspect ratio is 185/60.
- Radial mark - The next letter, if it's an "R" tells you the tire is a radial. If there's no "R", it's not a radial.
- Rim diameter - The next pair of numbers is the diameter of the rim in inches. The tire in the illustration fits a 14 inch rim.
- The load index - The next number and letter pair lists the load index and speed. The load index indicates the tires carrying capacity. For instance, 89 = 1,279 pounds, 88 = 1,235 pounds, 87 = 1,201 pounds, 86 = 1,168 pounds, and 85 = 1,135 pounds. Load indexes for light trucks and cars range from 70 to 126
- The Speed Rating - The speed rating is the maximum speed for the tire. Here are the letters and what they mean:
L 75mph 120 km/h Off-road & light truck tires
M 81mph 130 km/h Temporary spare tires
N 87 mph 140 km/h
P 93 mph 150 km/h
Q 99 mph 160 km/h Studless and studdable winter tires
R 106 mph 170 km/h HD Light Truck Tires
S 112 mph 180 km/h Family sedans and vans
T 118 mph 190 km/h Family Sedans and vans
U 123 mph 200 km/h
H 130 mph 210 km/h Sport sedans and coupes
W 168 mph 270 km/h Exotic sports cars
Y 186 mph 300 km/h Exotic sports cars
- Treadwear - This next 3 digit number is a comparison between a standard test tire and this particular tire. They run the tires side by side around a 400 mile loop in West Texas. If the subject tire wears the same as the test tire it gets a 100 rating. If it does twice as well it gets a 200; three times and it gets a 300 rating. So the 220 rating above means the tire did 220% better than the test tire.
- Traction - The next data on the outer ring is the traction rating. There are four ratings.
AA - the highest rating is followed by A, B, and C. These grades are measurements are G-force ratings on asphalt and concrete under dry and wet conditions.
- Temperature - This is a temperature resistance grade. The grade is established by measuring a loaded tire's ability to operate at high speeds without failure by running an inflated test tire against a large diameter high-speed laboratory test wheel. The rating is an A to C rating. A rates the tire over 115 mph. B rates the tire 100 to 115 mph. C rates the tire 85 mph to 100. All US built tires must be at least rated C for temperature resisteance.
- Maximum inflation pressure - Follow the small letters around the edge of the rims. Starting below the tire name you will find the maximum permissible inflation pressure followed by the US DOT tire identification number the Tire ply, composition and materials used in manufacture.
- Radial or not - Moving counterclockwise, tire markings repeat if it's a radial, but this time spells it all the way out.
- Tube or tubeless - Most tires are tubeless, but on the outside chance it's not here's where it tells you.
- Made in ???? - Some tires which are made outside the US list their country of origin here somewhere before the DOT code.
- DOT Code - The Department of Transportation (DOT) code uses this format: XXXX XXXX XXXX. The first two letters designate where the tire was manufactured. The next two letters and numbers are a code for the tire size. There may be 4 letters and numbers next if the tire is sold in Europe. Look for an "E". If not sold in Europe, these number codes may be omitted. If you do not see 11 or 12 characters following DOT, you may have to look on the other side of the tire. The final group of four in this long number which may be continued on the other side of the tire tells you the week and year in which the tire was manufactured. "2417" would indicate a tire made in the 24th week of 2017.
- Tire Ply and Composition - Tire makers are required to list the materials and number of layers of each material used to reinforce the rubber. Here is a sample of how it may be printed on the sidewall: TREAD PLIES: 2 POLYESTER + 2 STEEL+1 POLYAMIDE SIDEWALL PLIES: 2 POLYESTER
- Maximum Cold Inflation and Load Limit - This final marking refers how much weight you can safely put on the tires. If you have four of the tire shown above with a load limit of 1300 lbs. per tire, that means the vehicle, passengers and load shouldn't exceed 5200 lbs. or a bit more than 2 1/2 tons. That's why some trucks use dual wheels over the payload bed. It's not for looks although a dually really looks cool. The extra two tires increase the load the tires can safely carry. That way you can haul a granite boulder or two without flattening your tires.