As the last post picture says , tyrepressure is all about load, speed , and camber-angle( alignment) .
I once got hold of the European formula to calculate it, and went running with it.
Came to know by surfing the net, that this formula is adequate for standard load and XL tyres, but the in America used power in this universal formula gives to low pressures, before 2005 ( then they swiched over to the European power in it, only for SL and XL), and there are many old lists hanging around on the net.
Another thing is that you probably have low aspect ratio tyres ( the 40 in fi 235/40ZR18) for wich the maximum load of those tyres, is calculated to high by the tyre-makers. This has to be compensated, by using higher pressure.
Calculated it rougly to be 25% more then the calculations would give.
Comfort and gripp are totally gone then , but this is only for the savety of the tyres.
Taking this all into account, you still can use a lower pressure when you are not fully loaded, the advice-pressure given on the doorstyle or somewhere else on the car, is for fully loaded ( then rear more weight then front) and maximum speed of the car. Higher when towing goes from the idea that then the Gross axle weight rating ( GAWR) rear is exeded often, wich is not allowed but happens in practice. Front axle seldomly gets overloaded.
Goal of all the calculations is to keep the deflection of the tyre the same as when maximum load and reference-pressure on it ( wich is lower then the maximum pressure, depends on the tyre-type)
So give me the data of your tyres like maximum load and tyre-kind ( Standard load or XL/Extraload/reinforced, all on the sidewall) and the GAWR's of the car. Also the exact sises for the AR ( aspect/ratio)
Then I will give you a balanced advice for fully loaded.
If you can produce a Empty weight and configuration of the car ( motor in front or back and front or back drive , number of seats ) and how you normally use it, I can give a more balanced advice for normal use.
You may also try it yourselfes with use of spreadsheets on my skydrive.
https://skydrive.live.com/?cid=a526e...E092E6DC%21128
But they are older spreadsheets in wich I did not work out the latest developments yet. So partly I have to do it by hand.
This advice can then be used by others with the same model.
Greatings from Holland
Peter