Tire pressure advice is all about load on the seperate tires, but mostly axle loads are used.
I determined by reactions that if you have 85% of the load, that the pressure is calculated for, in real on a tire , that below that discomfort begins by bouncing.
for trailers I dare to go as low as 80% before screws tremble out of the woodwork.
Then front axle does not chanche much unloaded or loaded , because you simply cant put many things in the cabine, exceptions , verry heavy persons and motor on the front bumper.
Even the weight of the luggage is mostly even behind the rear axle wich lifts up the front.
So if we calculate the pressure for the GAWR ( give alternative word reading
, correction Gross axle weight rating) +5% the minimum weight if equally devided ( never so) can go as low as 85% of 105%=89,25% , comfort is still acceptable. Example GAWR front 4000 lbs then if real axle weight is 3600 lbs comfort is still ok.
Rear is another thing , loaded and unloaded makes a big difference.
Though you are not allowed to go over the GAWR motorhomes do often go over that , but normal lighttrucks , wich is the case here, stay under that too.
So for rear you have to use different pressures unloaded and loaded because it cant be done with one pressure to keep the loadpercentage ( as I call it L%) between the borders of 85% and 100%.
Because driver and co driver are in front, the rear bumping is not noticed that bad. So I would calculate pressure ( with my spreadsheet for motorhomes) for GAWR/0.85 to give no bumping when excact GAWR and leaves as much possible reserve for little overloading ( 18%).
Its never bad for the tires to keep at 80 psi ,only for your comfort but also important gripp.
To make this F( for ferry
) long story short, let me calculate so give maximum load of tires and GAWR's . Dangerous quess of mine is front unloaded front 50 and rear 60, and loaded front still 50 and rear 80.
But if calculation gives 90 that is allowed by the tire-makers , because this 80 AT-pressure is not the maximum pressure of the tire, as is thought by many.
For SL ( standard load) /P-tires this maxloadpressure is 35 psi for american tires, and XL/reinforced/Extraload 41 psi . Those tires only give maximum pressure on tire between 44 and 60 psi . From C-load and up ( C-load 50 psi) they give the AT-pressure/maxloadpressure/reference-pressure on sidewall and the maximum pressure shal be about 1.4 times the Pr ( wissle sound
oh no Pressure reference), I concluded of a PDF of Semperit( Continental).