Hi Jamie,
Thanks for coming onto the forum! :)
Apologies for my ignorance if this is a stupid question but it's something I've always wondered and I don't think it's been asked before.
With regards to tyre pressures, on most vehicles now (including my IS) there is a sticker on the inside of the door detailing manufacturers recommendations.
As they usually offer advice for different tyres sizes and loads, is it really worth changing the pressures for the different loads or is it just a case of setting it to your most common driving conditions. For example, usually it's just me in the car or me and the other half, but I do the odd fully laden motorway trip with 4 adults and luggage. Is it worth upping the tyre pressures just for these occasional times and then adjusting again after, or does it really matter that much? Am I over thinking this?! :D