First, you need to know how much weight is on your tires. Then you can calculate the pressure for each tire from the ratings on the tires.
For example: If your van weighs 10000# with 5500# on the rear and 4500# on the front, evenly divided side to side and your tires are rated at 3200# @80 psi.
Divide the tire rated load by the rated pressure to get a pressure capacity.
Divide that number into the load to get a pressure for that tire.
From the example:
3200# divided by 80psi = 40#/psi
for a 2750# load on a rear tire, 2750 divided by 40 = 68.75psi
for a 2250# load on a front tire, 2250 divided by 40 = 56.25 psi
A lighter van needs less pressure, a heavier van more pressure.
Higher pressure is safer - less flexing, less heat buildup.
Lower pressure may ride softer, but may 'wallow' more in turns.
Not know your weights and tire ratings, I would guess your pressure are close, but maybe a little low.
My van weighs 9800# has 275/70-18E tires and runs 65 psi all around.
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