I've just came across an article talking about changing tyre pressures to suit a change in tyre (wheel) size. The thinking being that a reduction in tyre sidewall size increases its stiffness, and therefore increases its load capacity, thus less air is required to maintain the same load capacity.
The proposal is that one should look up the load capacity of the original tyre by its required pressure and reference this against the load capacity of the new tyre size where the required pressue would be listed.
For my car, I've moved from the original 205/50/16 to 235/40/17 and so I'd need to look up the load capacity of the 205/50/16. According to the website, this is 1277lbs
as the stock pressure is listed as 39psi.
Cross referencing this 1277lbs load capacity against the pressure required by a 235/40/17 gives me 35psi
Given that this calculation doesn't factor in the differing tyre walls across different tyre brands and models, this could easily acount for the bone shattering ride some owners have complained of over the years when running larger diameter wheels... Throw in heavier wheel and tyre combinations, stiffer sidewalls and stiffer suspension and its only going to get worse.