Everything you need to know about tyre pressure

The inflation pressure is nothing other than the quantity of air which we are able to compress within the tyre carcass in order to allow the tread to ensure adherence to the road surface. It is measured in “bars” (1 bar equates to 1 kg/cm2), whereas the Americans prefer the term “PSI” (pounds per square inch), which corresponds roughly to 0.06 bar. It is precisely for this reason that no single pressure exists for vehicles in the same category, but on the contrary different values are always recommended for the two axles, which in turn vary depending on the weight they have to carry.

Seemingly minimal variations, that is to say from 0.2 bar too little or too much, have direct repercussions on the car's behaviour both during braking and while cornering, as well as influencing running costs, as they can generate an accelerated and irregular level of wear for the tread, as well as increasing fuel consumption. These are sufficiently valid motives to justify a level of attention which is often overlooked, given that for the vast proportion of drivers, tyre pressure is only checked on the occasion of the main summer holiday departure.

The evolution in the construction technologies for wheel rims and tyres has allowed manufacturers to ensure that the adhesion between two parts constructed from different materials has become more and more airtight; however, every tyre loses around 0.8 bar per month through natural causes. Thus, without even taking into consideration minor perforations, or air losses through valves or valve caps, it is obvious that after a period of only three months the wheels lose their ideal inflation level. And of course, this does not occur in a uniform manner, as readings can be very different as between one side or one axle and the other.

Whether the tyres are filled with compressed air or with nitrogen, their internal pressure does not remain constant, since gases are sensitive to variations in temperature, which make them contract or expand. Other conditions being equal, this means that a tyre will be more inflated during summer, and on the contrary will provide readings of a lower value in the event of particularly harsh climatic conditions. The inflation levels officially recommended by the car's manufacturers are traditionally set on the basis of readings to be taken at ambient temperatures. For this reason, it is advisable to check pressures on the basis of “cold” tyres, that is to say those which have not moved for at least two hours or which have not travelled more than five kilometres at a very modest speed. If this is not feasible, and you remember to check the pressures only once the journey has begun, it is necessary to add empirically 0.3 bar to the reading provided, in order to compensate for the expansion of the gas. But it is clear that you will need to attend to a correct measurement once the tyres have returned to a temperature which has not been conditioned by the load they have borne and by their movement over the road surface.

The most obvious effects of an incorrect alignment of pressure to the ideal values occur in the case of under-inflation. A 20 per cent reduction in the air inside the tyre reduces the distance achievable by the tread by the same amount. And at the same time, it produces an increase in the wear on the tyre's shoulders, which ensure adhesion through corners, a worsening of acoustic pollution and an increase of at least 3 per cent in fuel consumption, polluting emissions and CO₂. Thus, if a tyre has, let us say, an average life of 40,000 km, around 8,000 of these will be lost simply because of the laziness of not having ensured a regular top-up to the correct pressure. Excessive inflation too has negative consequences: as well as faster wear in the centre, it also results in reduced adherence, especially in wet conditions, and comfort levels can also be significantly compromised.