Tire sizes: know them so you don't make mistakes

Finding yourself having to replace one or more tires can be disorienting. Mainly for the variety of products but also on the infinite number of tire sizes available. A good place to start is by checking the tire size mounted on the vehicle, indication that is located on the “shoulder” of the tire (the external and visible vertical part of the tyre) also checking that they are correctly reported in the registration document. A check that never hurts, to confirm that we are using the right tyres, avoiding any problems with the police during a routine check.

If you then decide to change size to improve comfort or driving precision you will certainly need to adopt a new set of rims to accommodate the new size chosen which can be with lowered shoulder and rims of larger diameter for greater precision or alternatively diameter of the smaller rim and higher shoulder of the tire for greater comfort or to safeguard tires and rims if we travel on very rough roads with potholes.

How to read tire measurements

Various acronyms and symbols are embossed on the shoulder of the tire. Among these there are also the tire sizes and performance as an example 205/55 R 17 91 Vthe essential information to provide to the tire dealer or to get a quote.

Example size 205/55 R17 91 V

Tire width and height → 205/55

There nominal tire width, is represented by the number 205, a measurement expressed in millimetres. That is, the tire is wide from shoulder to shoulder 205 mm (20cm). The second number instead represents the height of the tire expressed as a percentage compared to width: 55 indicates that the height of the sidewall, between the top of the tread and the contact point on the rim, is equal to 55% of the width of the tire section, i.e. 205×55/100 = 137.5 mm (13.75 cm)

Rubber structure and diameter → R17

Continuing through the example code, you will find the letter R: radial. In general this letter represents the internal structure of the tire. To follow the 17, which represents the measurement of the outer diameter of the rim in incheson which the tires will be mounted.

Tire load index and speed code → 91V

The value ofload index of the tire in this case is “91” value which may be higher than that provided for in the registration document but not lower. This index corresponds to the maximum number of kilos that a single tire can support, a maximum of 615 kg. (91 corresponds to 615 kg as per the load index values ​​table).

LThe last letter “V” of the series represents the speed code, i.e. the maximum speed that the tire can reach in safe conditions. In the case of the example, the V indicates the speed category with limit a 240 km/h.

Pressure and DOT indications

Other indications that can be found on the shoulder are those concerning the inflation pressures of tires. These are established by the factory and can be found in the car's owner's manual, often preceded by the writing “max load” or “max press”.

Furthermore you can also find the DOT, to indicate the date and other information on the production of the rubber. For example, if the DOT were: DOT 4B 9Z 747R 3909.

• 4B code of the factory where the tire was designed;
• 9Z manufacturer's dimensional code;
• 747R optional code (manufacturer's own);
• 3909, seventh and year of tire manufacture.

Find out more about DOThere is an in-depth article.

Trademarks or other acronyms

In addition to the classic trademark present on any product, other symbols and acronyms can also be found. For example, you can find short alphabetical acronyms such as:

● C or LT, tires for commercial vehicles;
● XL, Reinforced or H, tires with greater load capacity for the same size. When replacing, they must be replaced with tires of equal capacity;

● T: Spare wheel tire for temporary use only.

Or you may notice some small representations. For tyres designed for'winter for example, the symbol of is used a mountain three peaks with a snowflake in the center. For the tires “Mud and Snow” instead the famous acronym “M+S”.

Tips for tire sizes

Specified both in the manual, on the shoulder of the tire and often inside the car door, the characteristics of the tire are fundamental both for the safety of the vehicle and for the law. It is in fact a risk of sanctions pecuniary if the measures do not reflect those approved for the vehicle or do not coincide with the guidelines of Traffic Laws. So be careful when purchasing how to read tire measurements. All this data can be processed more easily by a professional tire repair shop or a trusted tire dealer.

→ Speed ​​Code
→ Load Index
→ DOT of the tyres