Ever wonder what all those numbers on the sidewall of your tire actually mean? You’re not alone. Here’s a primer on tire size and other sidewall markings that can give you valuable information about your tires.(Click here to see the larger image.)
Width in millimeters – The first of the tire size numbers gives you the width of the tire from sidewall to sidewall in millimeters. If the number begins with a "P" the tire is called "P-Metric" and is built in the US. If not, the tire is a European metric tire. The only difference between the two is a very slight one in terms of how load rating are calculated for the size, but the two are essentially interchangeable.
Aspect Ratio – The aspect ratio designates the height of the tire, measured from the top edge of the rim to the top of the tire, as a percentage of the width. What this means is that the upper sidewall of the rim in this picture has a height of 65% of the 225 millimeter width, or 146.25 millimeters. To use this ratio to find the standing height of the tire for sizing purposes, see Plus and Minus Sizing your Tires.
Diameter – This number indicates the inside diameter of the tire in inches, which is also the outside diameter of the rim. If this number is preceded by an "R", the tire is radial rather than bias-ply.
Load Index – This is an assigned number corresponding to the maximum allowed load the tire can carry. For the tire above, a load index of 96 means the tire can carry 1,565 pounds, for a total of 6260 pounds on all four tires. A tire with a load index of 100 could carry 1,764 pounds. Very few tires have a load index higher than 100.
Speed Rating – Another assigned number corresponding to the maximum speed the tire is expected to be able to sustain for prolonged periods. A speed rating of V indicates a speed of 149 miles per hour.
Tire Identification Number – The letters DOT preceding the number indicate that the tire meets all Federal standards as regulated by the Department of Transportation. The first two numbers or letters after the DOT indicate the plant where the tire was manufactured. The next four numbers indicate the date the tire was built, i.e., the number 1210 indicates that the tire was manufactured in the 12th week of 2010. These are the most important numbers in the TIN, as they are what the NHTSA uses to identify tires under recall for consumers. Any numbers after that are marketing codes used by the manufacturer.
Treadwear Indicators – These markings on the outer sidewall show when the tire has become legally bald.
Tire Ply Composition – The number of layers of rubber and fabric used in the tire. The more plies, the higher the load the tire can take. Also indicated are the materials used in the tire; steel, nylon, polyester, etc.
Treadwear Grade – In theory, the higher the number here, the longer the tread should last. In practice, the tire is tested for 8,000 miles and the manufacturer extrapolates tire wear compared to a baseline government test tire using whatever formula they prefer.
Traction Grade – Indicates the tire’s ability to stop on wet roads. AA is the highest grade, followed by A, B and C.
Temperature Grade – Indicates the tire’s resistance to heat buildup under proper inflation. Graded as A, B and C.
The treadwear, traction and temperature grades collectively make up the Uniform Tire Quality Grading (UTQG) standards, established by the National Highway Traffic Safety Administration.
Max Cold Inflation Limit – The maximum amount of air pressure that should ever be put into the tire under any circumstances. This is an extremely misleading piece of data, as this number is not what you should be putting in your tire. The proper inflation will be found on a plaque, usually inside the driver's doorjamb.Inflation is measured in PSI (Pounds per square inch) and should always be measured when the tire is cold.
ECE Type Approval Mark – This indicates that the tire meets the rather strict standards of the Economic Commission for Europe.
There are also several markings which do not appear on this image, including:
M+S – Indicates that the tire tread is optimized for both mud and snow.
Severe Service Emblem – Also known as the ‘Mountain Snowflake Symbol’ because, well, it’s a picture of a snowflake superimposed on a mountain, this emblem indicates that the tire meets US and Canadian winter traction standards.
Knowing how to read the coded information on tire sidewalls can give you a big advantage when it comes time to compare tires to see which ones are right for you!