Tyre Sidewall & Tyre Size
How to Read the Side of Your Tyre
ASPECT RATIO
The relationship of a tyre's sidewall height to its section width.
DOT MARKINGS
A code (tyre identification numbers) molded into the sidewall of a tyre signifying that the tyre complies with U.S. Department of Transportation (DOT) motor vehicle safety standards. The DOT is followed by ten, eleven or twelve letters and/or numbers that identify the manufacturing location, tyre size and manufacturer's code, along with the week and year the tyre was manufactured.
LOAD-CARRYING CAPACITY
Indicates how much weight a tyre is certified to carry at maximum inflation pressure.
LOAD INDEX
An assigned number ranging from 0 to 279 that corresponds to the load-carrying capacity of a tyre.
OVERALL DIAMETER
The diameter of the inflated tyre, without any load.
OVERALL WIDTH
The distance between the outside of the two sidewalls, including lettering and designs.
SIZE
The combination of tyre width, construction type, aspect ratio, and rim size used in differentiating tyres.
SPEED RATING
This indicates the maximum safe speed at which a tyre is certified to travel under specified conditions. Speed ratings range from A (the lowest) to Y (the highest).
TYRE DESIGNATION
An alphanumeric code molded into the sidewall of the tyre that describes the tyre's size, including width, aspect ratio, rim diameter, load index, and speed rating. Most designations use the P-Metric system.
TYRE BRAND
This is the brand or manufacturer of your tyre.
TYRE PATTERN NAME
The tyre pattern name is the model or name designated to a particular tyre - this information is usually found after the manufacturer's name on the sidewall.
TYRE WEAR INDICATOR
Tread wear indicators ("wear bars") are located at the base of the main grooves and are equally spaced around the tyre. The tread wear indicators, which look like narrow strips of smooth rubber across the tread, will appear on the tyre when that point of wear is reached. When you see these wear bars, the tyre is worn out and it's time to replace the tyre. Always remove tyres from service when they reach a remaining tread depth of two millimetres (2 mm). Another easy way to check is to do the coin test. Take a five cent coin and place it with Queen's head down in the tread groove. If the tread covers the top of the Queen's head, then your tyres are OK. If the tread does not cover the top of the Queen's head, it is time to replace your tyres.
TYRE TYPE
This designates the type of vehicle the tyre fits. P is for passenger metric. Other letters are LT (for light truck), T (for temporary spare) and ST (for special trailers). If your tyre has no letter, it signifies that your tyre is a euro "metric" size.
TYRE WIDTH
The width of your tyre from sidewall to sidewall. In this example the width of the tyre is 225mm.
TYRE ASPECT RATIO
This identifies the tyre's aspect ratio, which is the relationship of the tyre's sidewall height to the tyre's width. In this example, the sidewall height of the tyre is 55% of its width. The lower the ratio, the smaller the sidewall height, which means better cornering, but a rougher ride.
TYRE CONSTRUCTION
This is the tyre's internal construction, which is "radial." Almost every tyre on the road has radial construction, which means the cords of the carcass plies inside the tyre "radiate" directly across from one side of the tyre to the other. Other letters used are D, for diagonal construction, and B, for belted.
WHEEL DIAMETER
This number (in inches) indicates that the tyre is designed to fit on a wheel with a 18-inch diameter.