P205/55R16 91W

P: The letter P means the tire is built to "Passenger-Metric" (American) standards and conforms to Passenger-Metric load and inflation tables. If there is no "P" at the beginning (ex: 205/65R15 94H) then the tire conforms to "Euro-Metric" or European standards and have different load and inflation tables that need to be followed. Other designations are LT for Light Truck, ST for Special Trailer, and T for Temporary (restricted usage for "Space-Saver" spare wheels).

205: This number is the width of the tire in millimeters. This is the most important number to describe a tire's contact patch.
For example, with everything else being equal, a P215/50 R17 tire (215 mm wide, 50% aspect ratio, Radial, 17-inch wheel) may look great, but a P245/50 R15 tire (245 mm wide, 50% aspect ratio, Radial,15-inch wheel) will out-corner it every time, because the contact patch is wider.

55: This is the tire's aspect ratio. This is the height of the sidewall from the rim to the tread, expressed as a percentage of the tread width.
In our example, the tire is 205 mm wide, and its aspect ratio is 55, then the sidewall is about 112.75 mm tall (55% of 205 mm). A tire size P 235/50 R15 would have a sidewall height of 117.5 mm (50% of 235 mm).

R: The R indicates the tire is a radial design. The letter B means belted bias, and D stands for diagonal bias construction. Virtually every new tire is a radial, unless you have a special tire for a classic car or a racing car.

16: This is diameter of the wheel this tire fits. While all the other numbers are metric, most wheels are still measured in inches. The number 13 would mean the tire fits a 13 inch diameter wheel, a 14 would mean the tire fits a 14 inch diameter wheel and so on.

91: This is the load rating of the tire. The actual weight the tire can handle isn't the same as the load rating number . For example, a tire with a 91 load rating can handle 1,356 pounds (615 Kilograms) of weight.  So, four of the tires used in our example can handle a total weight of 5,424 pounds (1,356 X 4 = 5,424)  (See table 3 below for Load Rating codes).

W: This letter is the speed rating of the tire. Speed ratings range from A1, which means the tires top speed is 3 m.p.h. maximum, to Y (ZR) which means the tires top speed is 186 m.p.h.. Tires driven faster than their rated speed can start to come apart.  The slowest rated passenger vehicle tire you should be able to find being sold in the U.S. is rated M, which is rated at a top speed of 81 m.p.h..  There are very few public road in the U.S. where you can legally drive faster than any tire's rated speed sold in the U.S. for passenger vehicles, but speed ratings are a very important safety consideration when shopping for tires.  (See table 1 below for Speed Rating codes).

Light Truck Tires

A light truck tire may have similar markings as a passenger car tire, with the letters LT in place of the P, such as a LT235/85R16, but light truck tires can also be labeled in a different way.

If you look at a tire that has 37X12.5R17LT on the sidewall, you would read it as follows:

Other Sidewall Markings
Treadwear: The treadwear grade is a comparative rating based on the wear of a tire when tested under controlled conditions. For example the useful tread on a tire graded 400 should last twice as long as a tire graded 200. However, the DOT allows each company to do their own testing, so another tire manufacturer may grade a comparable design 300, so a grade of 150 would last just half as long under their grading scheme. Because there is no set industry standard for treadwear, treadwear isn't very useful when comparing one manufacturer's grade versus the other.

Traction: Traction grades represent the tire's ability to stop on wet pavement as measured under controlled conditions on asphalt and concrete test surfaces. As of 1997, the traction grades from highest to lowest are AA, A, B and C. A tire graded AA may have relatively better traction performance than a tire graded lower, based on straight-ahead braking tests. The grades do not take into consideration the cornering or turning performance of a tire.

Temperature: Temperature grades represent a tire's resistance to heat and its ability to dissipate heat when tested under controlled laboratory test conditions. The grades from highest to lowest are A, B and C. The grade C corresponds to the minimum performance required by federal safety standard. Therefore, the A tire is the coolest running, and even though the C tire runs hotter it does not mean it is unsafe. The temperature grade is established for a tire that is properly inflated and not overloaded.

M+S: The "M+S" or "M/S" indicates that the tire has some mud and snow capability. Most radial tires have these markings; hence, they have some mud and snow capability.

Tire Ply Composition and Materials Used: The number of plies indicates the number of layers of rubber-coated fabric in the tire. In general, the greater the number of plies, the more weight a tire can support. Tire manufacturers also must indicate the materials in the tire, which include steel, nylon, polyester, and others.

Maximum Load Rating: This number indicates the maximum load in kilograms and pounds that can be carried by the tire.

Maximum Permissible Inflation Pressure: This number is the greatest amount of air pressure that should ever be put in the tire under normal driving conditions.

Max. Load Dual kg(lbs) at kPa(psi) Cold: This information indicates the maximum load and tire pressure when the tire is used as a dual, that is, when four tires are put on each rear axle (a total of six or more tires on the vehicle).

Max. Load Single kg(lbs) at kPa(psi) Cold: This information indicates the maximum load and tire pressure when the tire is used as a single.

Load Range: This information identifies the tire's load-carrying capabilities and its inflation limits.  The Load Range Letter on light truck tires indicates their ply rating (see table 2 below for Load Range codes).

Snow Tires: In some heavy snow areas, local governments may require true snow tires, those with very deeply cut tread. These tires should only be used in pairs or placed on all four wheels. Make sure you purchase snow tires that are the same size and construction type as the other tires on your vehicle.

More about Load Rating

While most people aren't concerned about a passenger car tire's load rating, if you are using a passenger car tire for pulling a trailer or carrying heavy loads in a large vehicle such as a van, you may want to get a tire with a higher load rating. In P-metric there is a standard-load tire, P205/65R15 92 and a "extra-load" tire, P205/65R15XL 95.  In Euro-Metric there are 3 different "standard-load" tires: 205/65R15 with service descriptions of 92, 93 or 94. The number doesn't translate into anything that would make obvious sense. You need to refer to a Load and Inflation Table for specific details. P-Metric and Euro-Metric tires use different Service Description numbers and have different load and inflation specifications For example a standard load P-Metric P205/65R15 92H tire carries a maximum load of 1,297 lbs while the Euro-Metric 205/65R15 94H tire is slightly lower at 1,290 lbs. Not a big enough difference to matter. A Euro-Metric 205/65R15 92H at 30 psi carries 1,213 lbs. You need to inflate it to 33 psi to match the load carrying capacity of the other two tires.

A P205/65R15 92H carries a maximum load of 1,400 lbs at 35 psi while a P205/65R15XL 95H carries a maximum load of 1,521 lbs at 41 psi. A 205/65R15 94H carries a maximum load of 1,477 lb at 36 psi. This shows that while it carries a slightly lighter load at 30 psi it carries a higher maximum load than the P-Metric standard load tire. A little care has to be taken when switching from Euro to P-metric tires and vice-versa.

*Current tire speed rating markings include the use of the service description to identify the tire's speed capability (P215/65R15 95V -maximum speed 149 mph).
**Any tire with a speed capability above 149 mph(240(kph) can, at the tire manufacturer's option, include a "ZR" in the size designation (P275/40ZR17). If a service description IS NOT included with the size description, the tire manufacturer must be consulted for the maximum speed capability(P275/40ZR17 - speed capability is greater than149mph). If a service description is included with the size description, the speed capability is limited by the speed symbol in the service description (P275/40ZR17 93W = maximum speed 168 mph(270kph).)
For tires with a maximum speed capability over 149 mph, tire manufacturers sometimes use the letters ZR. For those with a maximum speed capability over 186 mph, tire manufacturers always use the letters ZR.