Determining Tire Pressure

Trails And Tails wrote:
I have read that you should not pay much attention to the inflation psi marked on the tires, but rather go by the vehicle recommendation printed on the door edge. In other information I have found charts based on GVW. I'm just plain confused about this subject so I'm asking for some advice. I have 8 ply Goodyear tires that are stamped for 80psi. How should I go about getting the correct psi? TIA for your input.

The 8 Plyrating makes me suspect your tires are from European make , Goodyear also sels in Europe.
Michelin and Continental Vanco have in European marked also Cp tires.
Those are meanth for Campers as we call motorhomes in Europe.
They dont give AT-pressure of 65 or 70 psi belonging to 8PR, but give maximum allowed cold pressure of 80 psi .
The same as for normal car tires and XL, there also only maximum cold pressure is given and At-pressure/maxloadpressure is SL 35psi ( EUR 36 psi) and XL/reinforced/Extraload 41 psi( EUR 42 psi).

So you first have to determine what the right maximum load and AT-pressure of your tires are, before I can calculate an advice pressure for you. Then also give sises and speedcode and I will google if the maxload is in line with the given 80 psi max?/AT-pressure

Further I will give text I used for other fora for the other specifications fo Vehicle and tires , so I dont have to write it again and again.

Tirepressure advice is all about load on tire and speed ( and sometimes about alighnment - camber angle).

So if you can give details of car and tires , I can calculate an advice pressure with some reserve for things like, pressure-loss in time, unequall loading R/L, incidental extra load, misreadings of pressure scales,and misyudging of weight, etc.

This is from tires next and can be read from sidewall:
Maximum load or loadindex.
Kind of tire to determine the AT-pressure/pressure needed for the maximum load up to maximum speed of tire, or if lower 160km/99m/h/reference-pressure, wich is not the maximum pressure of tire.
Maximum speed of tire, most given as letter ( Q=160km/99m/h,N=140km/86m/h fi)
If you have offroad or tires looking like that , with large profile blocs that cover a part of sidewall, also mention, they are allowed lesser deflection then a normal road tire, then the tire maker used to determine the maximum load (to my conclusion the case for the Bridgestone tires on Ford Explorer in the Ford/Firestone affaire).
If you cant find all of it give sises of tire and Loadkind, then I will google for it.

From car next and mostly can be found on same plate as the original pressure advices:
GAWR and GVWR ( Gross Axle/Vehicle Weight Rating)
But best would be to determine the real weights in your use on seperate tires or estimate it as acurate as possible, by weighing per wheel(pair) or axle.
Maximum speed , you dont go over for even a minute in your use, eventually different for different situations, for instance when towing or fully loaded.This apart from trafic regulations, if you drive faster then allowed give that speed. Nature punnisches with tire-failure, police only with a penalty.
Give all that and I will calculate and give a picture of one of my filled in spreadsheets in my answer.
If other then original tires, indead as is already answered other advice is needed, a stiffer tire ( fi C-load instead of P-tire) needs a higher pressure for the same load, or the other way around, has lower loadcapacity for the same pressure.
Greatings from a Dutch pigheaded self-declared tire-pressure-specialist.