You could use my motorhome-tire pressure calculator.
To be found in next map on my skydrive that belongs to my hotmail adress with same username as here.
Motorhome tire pressure calculatorFirst download the spreadsheet by
RIGHT-CLICKINGand then choose download.
The spreadsheet uses the real loads or GAWR's and does not mind what tires where before on the car.
Mind that a stiffer tire as your 80 psi tire is , needs a higher pressure for the same load as a less stiff tire like probably your old where whith their 35 psi pressure needed for the maximum load.
If you cant work it out , give me the details , and I will do it for you.
50 to 55 could be the answer but I dont know wich loads you used.
Most certain you used a list wich gives optimistic loadcapacity for the pressure , as most American lists give.