Tyres at a glance

The elasticity of tyres, mainly made of rubber, provides the necessary functions to ensure comfort and safety, but there are trade-offs, the main being absorption of energy opposing to the vehicle movement, called “rolling resistance. The lowest rolling resistance is only obtained with no tyres, as in the wheels of trains and trams.

An infrared image of a test of rolling resistance on a truck tractor: the heat generated by the hysteresis is clearly visible (Hyttinen et al. Effect of Ambient and Tyre Temperature on Truck Tyre Rolling Resistance. Int.J Automot. Technol. (2022)).

As a tyre flexes when it rotates, friction between molecules causes energy to be converted into heat: this is known as hysteresis loss. Such loss accounts for the bulk of the rolling resistance (over 90%), the remaining being due to adhesion and sliding between the road and the tyre.

The forces involved in the deformation of a rolling tyre. (Source:  J. Hyttinen, Modelling and experimental testing of truck tyre rolling resistance),  

Rolling resistance is highly responsible of vehicles energy consumption: for example it accounts for about 40% of a diesel truck energy consumption, but with the increased efficiency coming from more aerodynamic cabin design, from autonomous trucks and from electrification, it could relatively increase to 60% or more. In any electric vehicle, moreover, lower rolling resistance would result in more energy recovered during the recuperation (regenerative braking), an additional further gain in range and contribution to the decrease of battery size!

As a battery electric vehicles is about three times more efficient than a similar internal combustion engine (ICE) vehicle, a poorly rated tyre in terms of rolling resistance has three times the negative impact: e.g. changing from an A-rated tyre to a C-rated can reduce range by over 8%. Rolling resistance of tyres, consequently is getting increasing attention, as involving:

• For the general consumer, liters of difference in gasoline consumption or tens of kilometers of range for every refuelling/recharge
• For the business, an important and relatively increasing voice of cost of road transport
• For the environment and all of us, a relevant cause of energy use and additional pollution.

It must be clarified that rolling resistance is finally influenced by different factors:

• Tyre design: for this, EPREL can help you chose the most efficient tyres
• Road design: e.g. a very noisy asphalt involves higher energy use (noise production as well involves energy use)
• Weather conditions: a wet surface or, worse, a snowy one, considerably increases the resistance to advancing of the vehicle
• Vehicle design: improving aerodynamics and decreasing weight helps reduce energy use (i.e. a SUV consumes far more energy than a station wagon with the same motor and comparable internal space).

Main forces opposing vehicle motion (Courtesy of Goodyear)

The rolling resistance is rated in the label in 5 classes. Rolling Resistance Coefficient” (RRC) limits for every class are indicated in a table  (from Annex I or the Tyre Labelling Regulation). 

‘Wet grip’ is the tyre’s ability to stick to the road in wet conditions. Ensuring safety is a crucial aspect: adherence and control of vehicles on a wet, slippery road is a frequent necessity, at all latitudes. A slick tyre has the best possible adherence on a dry road, but makes the vehicle increasingly uncontrollable  with water on the road, for increasing speeds, leading to the so-called "aquaplaning", i.e. a situation of complete lack of maintaining the direction of the vehicle. Tyre tread design is specifically intended to ensure water evacuation, but also the tread depth and the compounds are relevant. 

Safety on a wet road is rated in the label in terms of "wet grip". The test to grade the tyre is performed by launching a vehicle on a wet surface: the shorter the distance for completely stopping the vehicle when the brake pedal is fully pushed, the best the class in the tyre label.

Testing wet grip, i.e. distance covered to stop the vehicle on wet asphalt (Courtesy of IDIADA).

The wet grip is rated in the label in 5 classes, focusing on one aspect of wet grip, i.e. the wet braking performance. You may assume that each class is equivalent to about 3-4 meters, making a huge difference in some situations. Wet grip index limits for every class are indicated in a  table (from Annex I or the Tyre Labelling Regulation). Each wet grip class involves a few extra meters necessary to stop the vehicle on a wet surface.

Wet grip may degrade with the aging of the tyre but especially with wear (tread depth diminution): for this reason a new minimal requirement has been set in the legislation for wet grip in half-worn tyres. 

Aquaplaning: when it occurs (Courtesy of TireAmerica)

Abrasion is the unavoidable loss of rubber material during rolling and sliding contact of the tyre with the road, which results in the peeling-off of rubber debris (particulate matter).  Most of debris is encrusted to with other particles from asphalt: mostly deposited onto the road surface, road runoff plays a crucial role in spreading these "microplastics". These particles interact with water and air, triggering degradation and concomitant release of components such as tens of different organic and inorganic additives: the tire particles and the leachates derived from them during their degradation in the environment can cause biological responses in aquatic and terrestrial biota.

Mileage, or, rather, kilometrage, is an indicator of the durability of tyres in terms of total distance run until the tread depth reaches the minimum allowed by safety legislation (i.e. 1,6 mm in passenger cars). A short mileage results in additional waste to be treated (for the whole tyre carcass) and in a higher lifecycle cost for an earlier tyre replacement (to which a cost for mounting, balancing and vehicle unavailability must be added).

According to a consumer survey of 2019, mileage, with rolling resistance, are the most relevant parameters in customers purchase choice for replacement tyres, after wet grip, as both involve relevant money savings. 

Mileage and abrasion indicators will be added to the label, as soon as suitable testing methods will be available (work in progress), as foreseen by the tyre labelling Regulation. 

All tyres produce some noise but snow tires and off-road car one’s sound louder, as well as the run-flat, because of the stiffer sidewalls. 

External rolling noise measured level is expressed in A-weighted decibels -dB(A)- that is the relative loudness of sound as perceived by the human ear and it is indicated on the label with a pictogram symbol of a loudspeaker. Noise classes are 'A', 'B' or 'C’ , however the C class is almost not populated and the B class is the most crowded (i.e. most of tyres are in the B class, you can see the situation by querying EPREL). The class is not directly determined by the emitted noise level but depends on the limits according to the UN ECE Regulation 117 (a table with the limits is not simple, as depending on the year of approval of the tyre). The set limits are different depending on the tyre class (i.e., passenger car, van, or bus/truck). For passenger cars, the wider the section the higher the noise but also the limit set. An allowance of 1dB(A) is provided to winter and to XL tyres. Similarly for bus/truck tyres, different limits are set for traction and for other tyres, for normal and for snow tyres. The noise limits became more stringent since January 2023.

Testing of external rolling noise (Courtesy of IDIADA).

For testing the “external rolling noise”, a sound level meter measures noise levels: a microphone is positioned at the height of the ears of an average tall person and the vehicles passes by 8 times at speeds around 80 km/h for car and van tyres and around 70 km/h for bus/truck tyres.

Rolling resistance, noise, wet grip and wear are competing properties, e.g.:

• Hysteresis is negative for rolling resistance, but positive for grip and traction/handling;
• Lower rolling resistance may be negative for mileage or wear. 

For example, rolling resistance may be reduced by diminishing the tread depth,  but this, in turn, may result in less mileage, i.e. less kilometers run by the tyre before it reaches the minimum depth allowed by safety norms to ensure sufficient grip on wet roads (i.e. 1,6 mm).

The multidimensional complexity of competing tyres properties (Source: Sustainable mobility - Elsevier - Javier Araujo-Morera et al.)

For this reason, the tyre label is aiming at providing a balanced information for the purchase choice. Wet grip is the second parameter rated in tyre labels, so that the customer is guided in choosing the tyres that excel both in rolling resistance and in safety. A mileage class should be added, once a suitable tyre testing method has been agreed.

Regularly check tyre pressures: low pressure increases fuel consumption, affects braking performance and vehicle responsiveness and causes irregular and premature wear. Optimal tyre pressure is prescribed by the vehicle manufacturer: usually it is indicated in a sticker inside the fuel tank door or in the driver’s door frame (as well as in the vehicle manual). For many vehicles a different pressure is prescribed if at full load or when carrying the driver only. 

Comparing different inflation levels.

A few use recommendations to ensure a longer life of tyres :

• Internal tyre pressure increases with the temperature: measure the pressure after the vehicle has been not in movement for hours, as it increases after a few kilometres of travel (because of hysteresis). Recall that in winter air pressure decreases, thus you have to inflate more your tyres;
• The highest the pressure, within the prescribed limits, the lowest the hysteresis and thus the kilometres run using the same energy;
• Avoid harsh acceleration and deceleration, to reduce both both fuel consumption and tyre wear;
• Extra weight increases not only fuel consumption but also tyre wear; regularly clear out non-essential items from your car.
• Check for signs of uneven wear on the tyres, especially along the edges of the front tyres. Uneven wear could indicate a tracking or camber problem that could increase fuel consumption and cause your tyres to wear more quickly. 
• Camber is the term for how far the wheel tilts outward or how far it leans inward toward the vehicle: a wrong camber angle causes a ring of wear towards the inside or outside edge of the tyre, so check your tyres in this respect;
• Tracking, or wheel alignment, is a mechanical adjustment of the vehicle suspension system  to ensure that wheels turn in the correct position. A constant pulling to one side on the straight may be a symptom of incorrect alignment: wheel misalignment causes irregular wear and premature replacement need;
• If you periodically rotate tyres, check the tyre uneven wear first, or you would lose the information about which wheel or axis needs to be checked;
• A vibration of the steering may be a symptom of incorrect equilibration of the tyres.

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;
• contruction (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:

• S or F (Steer or Front): these tyres need to be in particular good conditions as controlling the direction. 
• D (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)
• T (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
• Z: (any axle): tyres can be also 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.

Range: indicates the intended use as:

• Long Haul
• Regional
• Urban
• On/Off road (e.g. for construction/material extraction sites) 
• Off road (not in the scope of the Tyre Labelling Regulation)

Types of tread pattern (from left to right: Long Haul, Regional, Urban, On/Off  (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:

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 hwn replcing your tyres.

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:

The rolling resistance is rated in the label in 5 classes. The following table indicates the “Rolling Resistance Coefficient” (RRC) limits for every class (as indicated in Annex I or the Tyre Labelling Regulation).

Back to rolling resistance section

The wet grip is rated in the label in 5 classes. The following table indicates the “wet grip index” limits for every class (as indicated in Annex I or the Tyre Labelling Regulation).

Back to the wet grip section

The speed rating on tires 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 

Example of OE markings:

_{Courtesy of} Michelin

Back to other indicators section