Tyre Pressures

Tyres will take a large range of pressure variation because they have to. Pressures are all about the pressure the tyre is expected to see in service and cold pressures are just a starting point as to where they are expected to get in service. Obviously for the vast majority of road driving, cold pressures are the best and sometimes only way to work this. However don't think that is what it is about. If you set 26psi in 15 degree ambient then when as recently the temps dropped to -8 that pressure will be much lower. Equally just the sun on a tyre can raise static pressure by as much as 5psi
So the cold pressure is not the operating pressure. One of the biggest mistakes people make going on track, where the tyre is worked much harder, is dropping to silly low pressures. A hot working pressure on an MX-5 is around 32psi. That is hot off the track, not having stood for 10 minutes. It can go higher. High pressures are a potential indication of a tyre being over worked or the rubber compound not being suitable and over heating. Some drivers will run their tyres in wet race condition as high as 45psi. This is because the stiff sidewall then means the tread moves more heating that better, combating the water which is sucking temp out of the tyre. For sprints, especially hill climbs, set the hot tyre to start with as there is no time for a tyre to build temperature and come up to pressure. Even on whole lap runs of somewhere like Snetterton, still start with a higher than "cold" pressure because the compromise of taking 3/4 of a lap to get to the right pressure is not worth the floppy tyre on that first 3/4. Hot or operating pressures, is what it is all about.

I've stickied this thread and put quoted nickd's post because it is very good.