I feel this way about tire pressures.
Different tires, and loads, require different volumes of air.
The tires that came from the factory, let's say are Continental 185 /75
R14's, 2 ply 4 ply rating.
And let's say that the tires are rated for maximum load of 1200 lbs, at
32 psi.
And this is what VW agrees with, as this is their main supplier of
tires, so that is what they print on the labels in the door jam, of the
vehicle, with the exception  stated as to an overload situation, in
which the pressures should be increased, to handle the extra  load.
Now, let's say after the stock tires are worn out, and the PO decides
he/she wants to increase the load carrying capacity of the Van, or
highway stability or better fuel economy, whatever.
Now he/she has Dunlop 235 x70 R14's installed with a full 8 ply rating
with a load rating of 1400 lbs at 40 psi.
The door jam sticker for recommended pressures just went out the window.
Also each tire manufacture has different pressure requirements,& load
capacities,even if the tire was of the same size,& ply rating.
I also feel that a greater tire pressure in the rear, would only have to
be increased, depending on what you are going to be hauling around, or
towing.
The bigger the load, the more pressure required, is pretty much an
accepted rule of thumb.
The moral of the story---
Read the sidewall of your tires prior to loading them up with air.

Later,

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Terry 85 GL