We often get asked about tyre pressure and what should the correct pressure be. When we tell our customers to put more in that the car recommends we are often met with puzzled looks. The simple answer to why we do this is down to basic hot air - that is to say when you arrive at our shop your tyre's have already heated up, which correlates to expanding hot air.
As most high school graduates know as air heats up it expands. In a car tyre this can mean an increased pressure reading. When the car is not in use the tyre will cool down and the pressure will drop, hence why we always put slightly more in your tyres.
The team at tyresizecalculator.com have put together a great article about tyre pressure.
The "recommended tyre pressure" is the pressure specified by vehicle or tyre manufacturer. The recommended inflation pressure refers to the cold tyres before the car is driven and tyres warmed up (ideally overnight).
Furthermore, tyre manufacturers define the minimum cold inflation pressure for every vehicle. Minimum cold inflation pressure provides tyre main performances.
Recommended car tyre pressure in manufacturers manuals are for the cold tyres. It means that the vehicle hasn't been driven for a several hours before the measurement of pressure. Ideal time for this is during the night or early in the morning, before going to work, or when the temperature of the tyres equals the outside air temperature. It also means that normal ambient temperature of tyres is about +20 degrees °C.
Daytime inflation pressure, when the outside temperature is higher, means that the tyres are not completely cold. The pressure inside warm tyres after driving is naturally higher. So never reduce the pressure of warm tyres. Once they cool down, their pressure could fall below the specified minimum tyre pressure.
An interesting experiment was conducted by well-known Tire Rack
The first two tyres are mounted on wheels and left it overnight to have their temperatures equalize and stabilize. In the morning the tyre pressure is set to 35 psi (2.4 bar). The outside temperature, tyre temperature and tyre air pressure are monitored during the day:
Outside temperature changes: 67°F (19°C) to 85°F (29°C)
Initial pressure in the morning: 35 psi (2.41 bar)
| Experiment Result Table | Tyre 1 - placed in the shade | Tyre 2 - placed in the sun |
|---|---|---|
| Tyre pressure: the highest of the day | 36.5 psi (2.52 bar) | 40 psi (2.76 bar) |
| Increase in pressure: | 4.3% | 14.3% |
This experiment shows that the pressure in the shade rose by 4.3%, in the sun even 14.3%. The pressure at a temperature of 67°F (19°C) - approximately identical to cold tyre pressure, is equal to the tyre manufacturer's recommended pressure. If temperature tyre pressure in the sun even rose 14.3% (0.35 Bars - 5 psi), that does not mean that we should reduce the pressure on the tyre manufacturer's recommended pressure. In that case tyre would be underinflated in the next morning.
The tyre is also heated during the drive so the pressure difference between the cold morning and warm daytime temperatures is even higher. But that does not mean we need to inflate warm tyre to the recommended pressure.
Never reduce the pressure of warm tyres. Once they cool down, their pressure could fall below the specified minimum tyre pressure. The recommended inflation pressures are for cold tyres, when they are at a normal outside temperature of about +20 °C. Adjust the pressure to recommended manufacturer's tyre pressure in the morning when the temperature is about +20 °C.