Tyre specifications
Tyre-size is the specification of a tyre, that let us know how wide the tyre is, and what’s the suitable rim size on which it should be mounted and those sort of things. It is specified in a standardized format by manufacturers for easy identification and replacement. If you do not know where you might find the size of a tyre, you can look at the tyre of a car. You will find it moulded on the side of the tyre wall.

If we ignore the imperial system, which was used in the past, there are only two systems that are in use today across the world. The imperial system, that was used in the past may still be worth knowing if you have a vintage car and require to convert their tyre size to today’s system for tyre replacement. Some of the formats used in the era before 1970’s are mentioned below. Since it’s an imperial system all dimensions were mentioned in inches.

Imperial system - No longer in use today

7 x 12.5
(Tyre width x Rim diameter)
(All dimensions in inches)

30 x 10.5 x 14
Tyre height x Tyre width x Rim diameter
(All dimensions in inches)

25.4 x 10.4 R15 LT C
Tyre height x tyre width R=Tyre construction, 15=Wheel diameter, LT= Tyre class, C=Load range
(All dimensions in inches)

Since the imperial system was discontinued around 1977, it’s unlikely that you will find tyres with markings shown above. The two systems that are used today are, P-metric system used in North America developed by tyre and rim association (TRA), and in the rest of the world, the other system is used i.e. the metric system aka ISO system or Eurometric system developed by European tyre and rim technical organization (ETRTO). So, depending on which part of the world you live in, you may find tyre sizes written in either of two formats mentioned below.

P-Metric System aka American system - P175/65R14 04 81T

A tyre conforming to P-metric standard will have specifications mentioned in the above format.

P = signifies the vehicle class for which the tyre is suitable. The different classes are: P = Passenger car, ST = Special trailer, LT = Light truck, T = Temporary (Space saver).

 175 = Tyre width (in mm)

 65 = Aspect ratio i.e. ratio of height to width. The aspect ratio can be read like, section height is 65% of the tyre width. So, the larger the aspect ratio, greater will be the section height. Tyres with low aspect ratio are known as low profile tyres, while tyres with high aspect ratio are called as high profile tyres.

 R = The letter lets us know about the tyre construction. The different construction types are R= Radial, D=Diagonal, B=Bias.

 14 = Rim size (in inches)

 D or 8 = Load range aka Ply rating, specified with a letter A - N with corresponding values 2 - 24. An increment of 2 is observed between alphabets (Some manufacturers do not mention this and some mention it simply as a number, this is found on commercial tyres or light truck tyres mostly). Depending upon the load range, the load index value is provided by manufacturers. Depending upon the load range value, the tyre's load rating is provided. The inflation pressure used for different load ranges varies.

81 = Load Index aka load rating, it provides the maximum value in Kg that tyre is rated for. For the number 81, load value is 462kg. The index can be as high as 139 for which the corresponding load value is 2435kg.

T = Speed rating
Tyre markings in Eurometric system and their meaning

Metric system aka Eurometric system - 255/50R17 95H C

The Eurometric system is quite similar to the American system. The difference is that the preceding vehicle class is absent and in case if it’s a commercial tyre, letter C will be present at the end indicating that it’s a tyre for commercial vehicle.

A thing to keep in mind is that even though the load index for metric and Eurometric may be same their load carrying capacity would be different. This is due to the reason that the load index value for Eurometric tyres is specified for inflation pressure of 36psi while the load index value for P-metric tyres are specified at 35psi. Another thing to keep in mind is that even if the load index values of different tyres of P-metric system is same their load capacity would be different. However, for Eurometric tyres tyres with the same load index will have the same load carrying capacity.

A thing worth noting is that both systems are metric yet, the rim size is provided in inches. So one might argue if they are truly metric systems? Tyres are evolving and so are their standards, for accurate information, it’s recommended that you refer to the manufacturer's website. For latest guidelines, refer to the manuals provided at the links below.

The Tire and Rim Association, Inc.
ETRTO - ETRTO - Pneumatic tyres, rims and valves

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