On the contrary. I run at an intermediate pressure. More than the minimum but less than the maximum recommended for when fully loaded. About half way.

Over a period of time and after tyres have done a couple of thousand miles you can tell if they are running at the correct pressure. Tyre ware should be even across the tread, if there is less tread depth in the centre they have been run over inflated and if they have greater tread depth the outside they have been under inflated. ...

This is what I do.

As an extreme example, I ran my landcruiser tyres at 36psi (against the toyota recommendation of 29). I also rotate the tyres. When I traded in my landcruiser with 40,000 miles on the tyres the salesman commented that the tyres looked like new. I didn't tell him that they'd started with 11mm of tread.

My current sorento is run at 35psi, against a kia recommendation of 33psi. Again rotated regularly. All tyres are even at 30,000 miles.

I am always surprised at how much a tyre's pressure - measured 'cold' - varies with ambient temperature. It's necessary to top up as winter arrives and things start to cool down naturally. One might imagine that Boyle's Law means that a drop of 10°C (in about 300°K) would cause a drop of 1psi in 30, but it aleays seems more than that.

Where our Accord is parked, the nearside faces the south , the rhs is in shade.

So I try to check and adjust tyre pressures on a cloudy day.

If I check the tyre pressures after the sun has been on it, the LH tyre pressures are 1.5 to 2.0 psi higher than the RH, that's how much difference it makes.

Yes, tyre pressure does rise by 2-3 PSI on a journey, but surely this is why manufacturers advice that one should check tyre pressures cold … and the recommended pressure has already taken this rise in pressure into account.

Checking tyre pressures 'cold' is the only way to have a reliable basis for comparisons, as the amount a tyre heats up in use will depend on the owner's driving style and the duration of use. But 'cold' means 'ambient', which can also vary a good deal.

I am always surprised at how much a tyre's pressure - measured 'cold' - varies with ambient temperature. It's necessary to top up as winter arrives and things start to cool down naturally. One might imagine that Boyle's Law means that a drop of 10°C (in about 300°K) would cause a drop of 1psi in 30, but it aleays seems more than that.

That's because tyre pressure are measured ABOVE normal pressure - so 30 psi measured is about 44.5 psi absolute pressure

Does this really matter unless of course you are driving a highly loaded car

As long as the tyre is inflated to manufacturers recommendations then surely what's just as if not more important is whether the tyre is not actually losing pressure?

Bearing in mind most car handbooks give a huge difference (mine is well over 10psi) between the driver only in the car as load and 4 passengers plus a full boot-load of things, I'd say that some nuance is quite reasonable, taking into account the extent to which the car is loaded up and the distance to travel, as well as the outside temperature.

When I load up my car boot for my (self-catering) holiday, it is not only full, but is quite heavy, probably the equivalent to a person in weight, hence why I up the rears' pressure a tad, but not the fronts, as its only me in the car.

As shown above, just accounting for the variation between the external air temperature for summer and winter can make 3psi difference, and that doesn't take into account the effects of heating via direct sunlight, which in late spring and summer months can be considerable when compared to the effects of ambient air temperature alone.

Some people also advocate dropping tyre pressures about 10% when there's heavy snowfall, in order to get better grip. I've never done that myself, so maybe other backroomers can say, if they did, what effects were the result.