tire pressure is all about load on the tires.
And because your plancard pressures are much lower then the maximum of the tires , the original tires with lower maximum load might have been replaced with higher maximum load.
Then if you have now stiffer tires then the plancard pressures are given for , you need a higher pressure for the same loads.
As long as you did not weigh the rigg , you have to do with the GAWR's ( Gross Axle Weight Rating's) .
I made a spreadsheet , in wich you can give the needed data, and it calculates the needed pressure, with my own formula, wich is saver then that , the tire- and car- makers use.
I even add a reserve load , wich you can chanche if wanted .
Here the link to map on my skydrive with the spreadsheets.
To use , download it first by right-clicking on it then choose Download from the dropdown-box. Dont pick open in web app or Exel , much to complicated.
After download open in Excell or Open Office Calc to use it fully.
But if you give me the needed details , I will calculate.
Need "maximum load xxxx lbs/kg AT yyy psi/kPa( cold)" from tires and GAWR's of car or if you have them weighed axles or better 4 point weighing.
Tried to atach a xls spreadsheet to this post , but dont know if it is accepted in this forum.