The tyre code that appears on the side wall of a tyre conveys information concerning the dimensions of the product, its load capacity and durability at high temperatures and speeds, and above all the intended use of the tyre.
The markings of tyres are regulated in the ECE-R 30 standards, which are legally binding in Europe. These rules mainly cover the marking on the side wall of the tyre, from which key information on the product parameters can be read off.
The tyre code can be found on the side of the model.
185 is the width of the tyre. This value is the first part of the code, and is expressed in millimetres. Tyre widths differ by a step size of 10 mm, depending on the code, so examples of differences in the size might be 175/65 R14, 185/65 R14 and 205/55 R16. The actual width of a tyre usually differs from the information in the code on the side of the tyre; this depends mainly on the manufacturer’s preference. The number 185 denoting a width of 185 mm refers to the distance between the furthest point of the outer side wall and the analogous point on the inner side wall, when the tyre is properly mounted on the wheel. This distance is called the cross-sectional width of the tyre.
65 is the tyre aspect ratio. This is the value following the tyre width, which indicates the profile of the tyre. It tells the user what percentage of the width of the tyre is accounted for by the height of its side wall, expressed as a percentage. For example, in a tyre with the code 185/65 R15 the aspect ratio is 65, and so the height of the side of the tyre is 65% of the product’s width, which is 185 mm. This gives a height of 120.25 mm. Typical low values for the aspect ratio are 50, 45 and 40, and such models are called low-profile tyres. In the case of higher-profile tyres, the aspect ratio is usually not given; for example, a tyre marked 165R13 may have an aspect ratio of 80.
The letter R means that the tyre has a radial construction. These tyres contain layers of fibres running in a radial direction, across the tyre and at a right angle. The R in the designation 185/65 R15 indicates that the tyre has body layers running radially from the centre of the wheel. Radial tyres are the most popular tyres with drivers, accounting for 98% of the total number sold.
15 indicates the diameter of the wheel rim on which the tyre should be mounted. Manufacturers express this value in inches.