How to read the tyre parameters

TYRE PARTS NAMES
	Tread Tread grooves Crown belts Cushion Sidewall Radial plies Bead wire Inner liner Carcass belt Shoulder

When replacing tyres in a vehicle, the replacement needs to match the exact “tyre size”, that is related to the rim,  The “service description” may be different, but with limitations. But a number of parameters and characteristics may be necessary to select the desired tyre, thus knowing the meaning of the parameters to consider may be useful.

This section explains how to set the search filters in EPREL in order to retrieve the tyres matching tyre size and service description (see the picture for reference).

Passenger vehicles (C1):

Tyre size

• section width: it is measured in mm from sidewall to sidewall;
• aspect ratio: it is the ratio of the section height and of the section width, expressed as percentage;
• construction (or structure): R stands for radial, B stands for bias. The vast majority is radial;
• rim diameter: it is the total rim diameter, in inches;

Service description

• load index: it is an assigned number that corresponds with the load-carrying capacity of a tyre. The number relates to this table. The heavier a vehicle is, the higher the load index;
• speed index: it is the letter that follows the load index and indicates the maximum speed the tyre can service. This table indicates the maximum speed in relation to the letter. The rated speed has to exceed the maximum speed homologated for the vehicle, but for winter tyres and limited to the winter season, national legislations may admit a lower index.

The aspect ratio, or profile, refers to the section height as a % of the section width.

The following is an example of tyre size and service description indication for a common modern passenger car:

Commercial vehicles (C2 and C3):

Tyres for vans, trucks and buses have some additional parameters characterising them.

Axle: the tread pattern is different depending on the position where the tyre should be used:

• Steering (Front): these tyres need to be in particular good conditions as controlling the direction. 
• TRACTION (Drive): there tyres are designed to cope with the specific stresses and strains imposed by transmissions of engine and brake torque. They can be mounted in pairs (dual mounting)
• FRT (Free Rolling Tyres, Trailer): these tyres designed to cope with the specific stresses for high loads, scrubbing, high mileages. They can be dual mounted as well. FRT-marked tyres (UNECE Regulation 54, paragraph 3.1.15) are approved only to be fitted on trailer axles
• All positions: tyres can be also generic and not optimised for a specific axle.

Tyre codes based on the axle where they are designed to be fit (Courtesy of Michelin)

Drive tyres in a dual-mounting configuration.

Mission: truck tyres are designed considering the intended use as:

Mission Profile	Description	Typical Features	Common Application
Long-Haul	Designed for long-distance driving on highways	Fuel-efficient, durable, low rolling resistance, optimized for high speeds.	Long-distance trucks, freight vehicles, intercity transport.
Regional	For medium-distance travel between regions or towns	Mix of highway and urban conditions, high durability, moderate speed.	Delivery trucks, regional haulers, buses.
Urban	Designed for short-distance driving in urban areas.	Excellent grip, frequent stop-and-go, wet traction, comfortable ride.	Delivery vans, urban buses, light trucks.
Municipal	Tyres for municipal vehicles like waste collection and street cleaning.	Heavy-duty, robust for urban terrain, durable, high traction	Garbage trucks, street sweepers, utility vehicles
On-Off	Tyres designed for both on-road and off-road conditions	Aggressive tread pattern, durability for mixed terrain, high traction.	Off-road trucks, construction vehicles, mining vehicles.

Images: Types of tread pattern (Source: VIPAL band catalogue)

Nominal operating code. This is made up of the load symbol for single and twin tyre fitments and the speed symbol for the reference speed (e.g., in the picture 154 means 3.750 kg per tyre as a single tyre fitment and 150 means 3.350 kg per tyre in dual fitment and M stands for a max. speed of 130 km/h).

Additional service description: on tyres o trucks or buses, the permitted load can be increased, and the maximum speed reduced, or vice versa. An additional load index is then given for single/twin tyre fitments and a letter for the appropriate reference speed. The additional service description is encircled (e.g., in in the picture 156/150 L).

Main indications on a truck sidewall: size, service description, nominal operating codes and additional service description ( Courtesy of Uniroyal)

The following are some examples of tyre size and service description indication for a common truck tyre:

Other indicators

Tyre wear: the indicators are high ridges evenly spaced along the tread, set at 1.6mm, between the sculptures, to identify when a tyre is worn down and needs replacement by law (when the treads on are worn down to the same level as the wear indicators, the tyre has reached the minimum admissible tread depth level).

Tread wear bars in a new and in a completely worn out tyre (Source: guideautoweb)

Manufacturing date: it is moulded on the sidewall in a area called DOT, with a series of ovals only required by US legislation: the rightmost includes the indication of the week (beginning from Sunday) and year of manufacturing.

Week and year of manufacturing, circled (Courtesy of Michelin)

OE (Original Equipment) marking: Such marking on the tyre indicates that the tyre was designed by the tyre manufacturer and approved to be fitted as original equipment for a vehicle brand or even specific vehicles. The differences between tyres of these types might relate to vehicle handling, comfort, noise and sometimes or optimised rolling resistance. Check the list of OE markings if you wish to maintain the exact vehicle behaviour when replacing your tyres.

 Original Equipment Manufacturer (OEM) marking

Courtesy of Michelin

Tyres without any OEM marking are general replacement tyres, suitable to any vehicle. OEM tyres have characteristics perfectly matching the original equipment tyres that featured the new vehicle when new.  Car makers often set specific requirements for the featured tyre, involving e.g. specificities in design of belts, in the chemistry of tread compound, etc.  If you look for the exact tyre as originally mounted on your new vehicle, you should look for the same symbol on the sidewall when buying the replacement tyres. Not all new vehicles are equipped with specific OEM tyres. The following is a non-exaustive list of OEM marking you may recognise moulded on the tyre sidewall.

Mounting direction: tyres may require a specific mounting caution

• Directional: should be mounted in only one direction of rotation, or uneven tyre wear, increased interior noise and vibration levels may occur (the rotation direction is moulded on the sidewall);
• Asymmetric: the tread pattern is different for the side to be mounted internally or externally (indicated on the sidewall)

Directional and asymmetric tyres

Unrestricted mobility: road vehicles were traditionally provided with a spare wheel intended to reinstate vehicle mobility in the event of a puncture or similar damage. The spare wheel can be either of the same size of the units normally equipping the vehicle, or  of “temporary use”, thus for continuing the travel in restricted conditions (i.e. below 80 km/h for a max distance). In the light of the decreasing probability of a puncture (better roads, better tyres) and because of the general trend to equip cars with bigger and bigger wheels, modern cars are not generally equipped with a spare wheel.  In case of a damage, “tyre inflation kits” are available that not necessarily solve all situations. For example, these kits are unusable with tyres having a “noise reduction” system (a foam glued internally to the tyre, reducing the noise resonance) or when the damage is relevant. 

Three alternatives to avoid the spare wheel: Run Flat or Extended mobility, self sealing, and support ring tyres (Source: adapted from Sustainable mobility - Elsevier - Javier Araujo-Morera et al.)

Other solutions are now available for passenger cars and, to some extent, to vans that avoid to have to replace the tyre, possibly in a dangerous situation, such in a tunnel or high speed road:

	“Run flat” tyres (RF, self-supporting tyres)  have specially reinforced sidewalls to enable them to perform when deflated.  The driver can continue travelling without needing to stop to replace the deflated tyre. The sidewalls are stiffer than regular tyres. A letter “F” is placed before the rim diameter marking, e.g. 245/45RF19, and a specific pictogram is moulded on the sidewall to recognise these tyres.
	“Extended Mobility (EMT) tyres” are similar to run-flat tyres but may have a slightly different construction: they could either have reinforced sidewalls or use a special sealing technology that helps maintain air pressure temporarily. For both FR and EMT tyres the maximum distance and speed permitted under pressure loss may vary according to the tyre or vehicle manufacturer (typically max 80 km at max 80kmh).   The image shows the ISO symbols for Run-flat (top) and Extended mobility (bottom) tyres.
	"Self sealing" tyres may permit to continue the trip in case of a puncture from screws, nails or similar: a mastic sticks to the object causing the puncture, creating a seal around the object if it remains in the tyre. When the object is removed, the sealing mastic is “drawn” into the hole, sealing the edges. Sealants don’t fix the tyre forever and are only for emergency purposes. Use of sealants is controversial.
	“Support ring” tyre (RFT) systems employ a ring of hard rubber or similar material that can support the vehicle’s weight in case of air loss. This is mostly used in heavy vehicles, such as armoured limousines, vans for transport of valuable content or military vehicles. The image shows the ISO symbol for Internal Supporting ring.

Annexes (tables):

The speed rating on tyres indicates the maximum speed they can safely handle. 

Back to the service description section 

The load index corresponds to the maximum load (in kg) a single tyre can carry.

Back to the service description section