It looks from the figures posted that the pressures go up as the rim/mag diameter increases. Is this because the profile is assumed to be becoming lower or possibly the width becoming greater?
I imagine a wider tyre of a given rolling (outside) diameter will require the same area on the ground (contact patch, footprint) as a narrower tyre at the same pressure to support the car and therefore 'squash down' less. Hence requiring less pressure. Furthermore as the rolling diameter increases the increase in the contact area for a given width would increase at a greater rate for a given amount of 'squash' and again require less pressure.

What am I missing here as I have not doubt that the information posted by nannas_ed, Mad Matt and others is drawn from considerable experience.

Wear patterns would no doubt reveal the correct pressures in most cases but my attention span is not that long. Also as I like stirring s**t the 'suck it and see' principle leaves a bad taste.

I have 16/225 on my car and they look low at 32PSI.

I ride a bike too and pressures make a noticeable difference to the feel of the bike. FYI the pressures are around 34PSI for 18" wheels 220Kg bike and 36 PSI if I'm out with the lads.

Dazed and confused I am going to try 38PSI in my 16/225 car.

Cheers

So, if I have 18x8 rims (don't know the tyre specs off the top of my head), what would be a good pressure to run? I am assuming 40psi and up?

Anyone got a definitive number?

40psi. Measured when tyre is warm. Pay no attention to Cold tyre temps as different enviroments will greatly effect this figure. Take the car tor a dive to get tyres up to temp, a few kays should do it, say 5 kays, then them check you have 40psi in there. the standard operating temp is the one you want.
40psi, got that 40.

Wicked. Because I just went and pumped them up to that exact pressure.

lol

Thanks.

i have 225 50 16" and the tyre place told me to have it at 38 - 40 psi