Tire code
These codes specify the dimensions of the tire and its key limitations, such as load-bearing ability and maximum speed.
The code has grown in complexity over the years, as is evident from the mix of SI and USC units, and ad-hoc extensions to lettering and numbering schemes.
In practice, the standards of the three organizations have evolved together and are fairly interchangeable, though the load and inflation tables will give slightly different values for the same size tire.
A 2- or 3-digit number indicating the "aspect ratio" of the sidewall height as a percentage of the nominal section width of the tire.
There is the rare exception of metric-diameter tires, such as the use of the 390 size, which in this case would indicate a rim of 390 mm in diameter.
In some parts of the European Union, tires that are not fit for a car's or motorcycle's particular maximum speed are illegal to mount.
The sole exception are M+S tires, where a warning sticker stating the allowed maximum speed must be placed within clear sight of the driver inside the vehicle.
It is useful in identifying tires subject to product recall[17] or at end of life due to age.
After the size code, the next sequence in a DOT number consists of three or four digits that designate the tire type.
This coding, though optional, is widely used by tire manufacturers for internal tracking purposes, such as managing returns and quality control.
An (upper case) "E" indicates that the tire is certified to comply with the dimensional, performance and marking requirements of ECE Regulation 30.
A (lower case) "e" indicates that the tire is certified to comply with the dimensional, performance and marking requirements of Directive 92/23/EEC.
The number in the circle or rectangle denotes the country code of the government that granted the type approval.
Flotation tires keep vehicle tires floating above loosely packed dirt, minimizing soil disturbance in agricultural environments and maximizing vehicle stability on unpaved surfaces in construction environments.
There are numerous other markings on a typical tire, these may include: When referring to the purely geometrical data, a shortened form of the full notation is used.
Less commonly used in the US and Europe (but often in Japan for example) is a notation that indicates the full tire diameter instead of the aspect ratio of the side-wall height.
Whilst this is theoretically ambiguous, in practice these two notations may easily be distinguished because the height of the side-wall of an automotive tire is typically much less than the width.
The actual diameter of a specific tire size can only be found in the TRA Yearbook or the manufacturer's data books.
An aspect ratio of 78% was typical for letter-sized tires, although 70% was also common and lower profiles down to 50% were occasionally seen.
