New to this forum, found this topic searching for tirepressure .
Call myself "Dutch Pigheaded Selfdeclared Tirepressure-specialist, and story began when I got hold of the official European formula and went running with it.
If wanted I will introduce myself in a seperate topic, but dont think its needed.
Your formula probably is intended to use 25% instead of 35% , or you added the 10% for reserve, wich is not that bad.
Further it comes down to lineair calculation, so part of maximum load = part of maximum pressure .
The official European formula has a power in it, and American TRA switched of to that power as late as 2006 for radial tires, but only for P-tires ( in standard load SL, and XL/reinforced/Extraload) and left LT tires to the lower power wich gives more loadcapacity for the same pressure then European used power for all kind of tires ( up to trucktires)
You use maximum cold pressure given on sidewall of tire, but this is not the cold pressure the maximum load is allowed to be carried up to 160km/99m/h, and wich is called reference-pressure in the official formula, but will call it AT-pressure furtheron, because on LT and Trucktires this is given at sidewall only, and not the maximum cold pressure , mostly something like this "maximum load xxxx lbs AT yyy psi(cold).
for SL P-tires this At-pressure is USA system 35 psi, EUR system 36 psi but with exeptions possible.
For XL// P-tires AT-pressure 41psi in USA system , and in EUR system 42 psi again with exeptions.
So those values you need for Z in your formula.
You also assume the GVWR to be devided equally over the 4 tires, and that is not true.
In practice when only driver and a litte load , more weight on front axle then rear, so front higher pressure needed then rear. When fully loaded its the other way around, and you have to highen up the rear pressure but the front can stay about the same ( as a rule of thumb).
then advice given by car-makers is nowadays not based on axleloads anymore, often AT-pressure of tires in America.
For higher speed the official formula gives a system of highening up the AT-pressure depending on speedcode, and as soon as maximum cold pressure is reached , for V,W and Y speedcodes , the maximum load is lowered with a system depending on speedcode again.
Hope you have an idea now, but ask if new questions pup up.
In next post I can give link to my public map of One-drive that belongs to my hotmail.com adress with same username as in this forum ( so jadatis, like this to prefent spamm robots to combine my adress).
Mostly on first posts this is not allowed
Greatings from this Dutch Pigheaded Self-declared Tirepressure-specialist,
( ja dat is ) Peter ( from a Dutch name song from the 1960's, translates as "Yeah thats Peter")