Tyre pressure and TPMS: the answers

At least once a month and before every long trip or one with a heavy load. Tyres lose air slowly even when intact, so a periodic check is necessary even without warning lights. It is also worth checking when the temperature changes noticeably, for example at the arrival of autumn or winter. The check must be done with cold tyres, because driving warms the internal air and raises the pressure read.

Sources: michelin.ca

The correct pressure is the one given by the car manufacturer, not the one moulded on the tyre sidewall. You usually find it on a label on the driver's door pillar, inside the fuel flap or in the owner's manual. Different values are often given for front and rear, for the approved sizes and for normal or full load. The number on the tyre sidewall is instead the maximum pressure the tyre allows.

Sources: michelin.ca

It means measuring it before setting off or after the car has stood for at least three hours, as Michelin indicates. While driving the internal air heats up and the pressure rises, so a hot reading comes out higher than the real cold value. The values given by the car manufacturer refer to cold tyres: to compare them correctly always measure in these conditions.

Sources: michelin.ca

Never deflate a hot tyre: Michelin states that the higher reading is normal and corrects itself when the tyre cools. If instead the hot value is already lower than the one indicated by the manufacturer, the tyre is definitely under-inflated: add air and recheck the cold pressure as soon as possible. If the filling station is close, the best solution is to measure before setting off, with the tyres still cold.

Sources: michelin.ca

Yes: in the cold the air contracts and the pressure drops, in the heat it rises. As an order of magnitude, Michelin indicates about 1 psi (about 0.07 bar) for every 6 °C of change. That is why pressure should be rechecked at every change of season and when moving from a mild climate to a very cold one. A tyre that was perfect in July may be under-inflated in January with no leak at all.

Sources: michelin.ca

Rubber is not completely airtight: a small amount of air passes through the tyre's structure and, to a lesser extent, can escape from the valve and rim. That is why pressure drops gradually even without punctures: Michelin states that a tyre can lose up to about 1 psi (about 0.07 bar) a month. A much faster drop is abnormal and should be investigated: a nail, a faulty valve, a damaged or corroded rim edge.

Sources: michelin.ca

Michelin states that an under-inflated tyre wears faster on the outer edges, responds worse in emergency manoeuvres and increases fuel consumption. Correct pressure also reduces the risk of aquaplaning in the wet. In serious cases the tyre overheats and the internal structure can be damaged even without obvious signs outside. Since a moderate pressure drop is not visible to the eye, the only remedy is regular checking with a gauge.

Sources: michelin.ca, middle-east.michelin.com

With pressure too high the centre of the tread carries more load and wears before the edges: Goodyear points to central wear as a sign of over-inflation and links it to a greater risk of a blowout. The footprint shrinks and the tyre absorbs irregularities less, so comfort worsens. The solution is to restore the pressure to the manufacturer's value, measured cold, and not to use the maximum value moulded on the sidewall as a reference.

Sources: goodyear.eu, michelin.ca

It depends on the car: many manufacturers give two values, one for normal load and a higher one for full load, with holiday luggage or a trailer. You find them on the door label or in the owner's manual. If load values are not given, use the standard one and don't improvise. Once the car is unloaded, return the pressure to the normal value.

TPMS (Tyre Pressure Monitoring System) is the system that warns the driver, with a light or by showing the values, when the pressure of one or more tyres has dropped too far. Michelin notes that in Europe it has been mandatory on all new cars since 1 November 2014. It does not replace checks: according to Michelin the system generally steps in when the tyre has already lost about 20% of its pressure, about 0.4 bar, so the monthly check is still necessary.

Sources: michelin.co.uk

Direct TPMS uses a sensor in each wheel that measures pressure and sends it to the car, which shows a light or the values. Indirect has no pressure sensors: it infers the drop from the rotation speed of the wheels detected by ABS/ESP, because a flat tyre has a smaller diameter and turns faster. Direct often shows the values for each wheel; indirect generally has to be reset after adjusting the pressure or changing tyres, according to the owner's manual.

Sources: michelin.co.uk

It indicates that at least one tyre is below the specified pressure; on many cars, if the light flashes and then stays on, it instead signals a system fault (the exact behaviour is in the owner's manual). Stop as soon as you can safely, visually check the tyres and measure the pressure of all four with a gauge. If a tyre keeps losing air, go to a tyre shop before continuing.

Sources: michelin.co.uk

It depends on the type of system. With indirect TPMS you generally need to reset or recalibrate the system after bringing the tyres to the correct pressure, with a button or a command in the car's menu (procedure in the owner's manual). With direct TPMS the light usually goes out after a few minutes of driving. If it stays on after recalibration and with correct pressures, there may be a faulty sensor or one with a flat battery.

Sources: michelin.co.uk

The direct TPMS sensor is mounted on the rim, so it stays there when the tyre is changed, but it must be handled with care so as not to damage it. After fitting or a wheel swap, many cars require the control unit to recognise the sensors (learning procedure or 'relearn'), according to the owner's manual. If you fit a second set of rims, for example for winter, you need sensors on that one too. Michelin notes that with a different size the system must be recalibrated by a professional.

Sources: michelin.co.uk

Yes: at Pneumatici SuperStore, in Milan, you can have the pressure checked and inflated with nitrogen or with air. Michelin states that most tyres can be inflated with air or nitrogen, as long as the manufacturer's recommended pressure is respected, and that the two gases mix without problems. Whichever you choose, pressure must still be checked at least once a month and before long trips.

Sources: michelin.com.au

Nitrogen is not a special gas: the air we breathe already contains about 79% of it. Michelin explains that with nitrogen tyres lose air more slowly and hold pressure longer, but that most tyres can be inflated with either air or nitrogen, as long as the manufacturer's pressure is respected. So the benefit exists but does not replace checks: pressure must be verified at least once a month and before long trips.

Sources: michelin.com.au

Yes, the two gases mix without problems: if a tyre inflated with nitrogen needs topping up and there is only compressed air, you can use it. Nothing harmful happens; the possible benefit of nitrogen is simply reduced. What matters is that in the end the pressure is the one indicated by the car manufacturer, measured cold.

Sources: michelin.com.au

Yes, they must always be fitted: the cap protects the valve from dirt, sand and moisture that could get in and cause small air leaks. It does not hold pressure on its own, because sealing is handled by the valve's internal mechanism, but it is a cheap and important protection. If you lose one, replace it as soon as possible.

Rubber valves age and can crack, which is why it is common practice to replace them when fitting new tyres: they cost little and prevent leaks. On cars with direct TPMS the valve is often built into the sensor: in that case the sealing components specified by the sensor manufacturer are replaced. A cracked or leaking valve must be changed immediately, without waiting for the next tyre change.