A couple of days ago I checked the tyre pressures on our new car for the first time since buying it (several weeks ago). Obviously we want about the right pressure in the tyre to get it to do its job properly and to help get the best fuel economy out of the car. However, getting the pressure right isn’t an easy thing.
For a start, I’m sure every garage’s air machine reads differently. Certainly the ones with an analogue scale can be all over the place in their readings. Even two readings of the same tyre on the same (analogue) machine can be way different (I’m thinking about the one at our local garage). That might partly be because I’m not using it correctly – but I think I’m following the instructions.
But also the temperature of the tyre is going to make a reasonable difference to the pressure. Air is going to follow the ideal gas law – roughly anyway, which says that for a given mass of a given gas, pV/T is constant, where p is the pressure, V is the volume, and T the (absolute) temperature. For constant volume (OK, the tyre isn’t exactly constant volume) pressure would increase linearly with temperature. On a cold morning, before you’ve gone very far in the car, the air inside could be sitting at 0 Celcius, or 273 kelvin absolute temperature. On a hot day after an hour on the road, it might be more like 50 Celcius, or 323 kelvin. That’s a difference in temperature of 18%, and will correspond to a similar change in pressure.
However, there are other factors too. A well inflated tyre – on a hot day for example – will stretch the tyre and make it more likely to lose air through the tyre wall. So in summer, tyres will lose more air than in winter.
The petrol-head websites (e.g. this one) say, unsurprisingly, that pressure should be checked with a cold tyre – i.e. early morning with little driving done to get to the air hose. If you have the right pressure then, you won’t be under-inflated at other times. Plus, it’s as close to a practicable standard as you’re likely to get with a functioning car – no-one’s going to take them to a physics lab and leave them overnight in controlled conditions for an accurate measurement every month!