Signed in to this forum, to go worldwide with mij articles about calculating tire pressure.
I am from Holland so exuse me for mis-spellings and poor Englisch.

On the dutch camperforum the question of tire-pressure was asked often, but now not anymore.
I have got hold of a copy of the first page of the European Tire&Rim Technical Organisation 2007 standard manual.
On that page some formula's witch the big tire-facturies use to calculate the correct tire pressure, for normal car tires on the road.
I have simpliefied them to 1 formula wich normal people can use.
Under the next link you can find the car version and ideas for re-calculating when you have tires with chanched specifications. http://board.carstyling.net/showthread.php?t=67492
Sorry, it is in Dutch so someone has to translate. Luckily the formula stays the same.
So here it is so you can work with it .

A=B*(C/D)^1.25
A= the tire pressure you want to know
B= the maximum tire pressure,which you can find on the tire
C= The actual weight on the tire, the most discussable item.
D= the maximum load for the tire, also can be found on the tire in lbs, kg, or Load-Index( search in list on internet for the maxload then).
The minimum is 1,5 bar =??psi but I have seen in lists for car-tire pressure that 1,4 as minimum is also used.
And mind that there are 2 (somethimes 4) wheels on an ax .

To calculate what weight the tire can bear at a certain pressure you can use the next formula.
C=D*(A/B)^0,8
A= the actual pressure on the tire
B=same as above
C= The load the tire can bare at the actual pressure
D = same as above.
2^2 meens 2 square 2 = 4 on mathematical calculators, also that of windows calculator jou can find a button X^y.

Mayby there is someone on this forum with mathematical interest , who can work it out . You can send me a personal message for more information or place it under this topic. Mind that you have to build in a savety-factor. When you add 10% to the weight on the tire for calculating , the car stil rides comfortabel.
I am also prepared to give some examples here, give me the right information and I wil give you a full report.
Mind that ,though the tire-firm used the sises of the tire and some more data to determine the max weigh, the sises are not needed for calculating. Only Load index and maximum Pressure are needed from the tire, and can, almost always, be found somewhere on the tire.

greatings from Peter

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Been busy with Exel and made two forms for calculating new tire pressure, when the original advice pressures are known. One is for, if you also know the maximum axis-loads and the other if you only know the Maximum Permissable weight.
If there are words from whitch you say"We use an other word for that in Englisch", go ahead and make a better translation. Place it on your own site, and make a link under this topic. Thenothers dont have to wait until it comes the whole way from Holland. Or sent me the corrections.

It gives a good indication, but because the pressures of the original tires where rounded up to 1 psi at the first calculation,there is al little deveation possible. You can also fil in a form with 1psi lower normal pressures and a form with 1psi lower maximum pressures. The new pressures are in every case between thes last two forms.
It works perfect for normal cars,for special cars like those with twin-wheel-axis behind is stil in the Beta-phase.

One thing I did not include in the formula, and that is the camber factor, simply because I didn't have that part of the formula. You need this, when the wheels are not straigt on the road, not like this|-| but like this/-\.
This is mostly done with the rear wheels for better grip when you make a curve. For this you have to multipy the maximum pressure of the tire with a factor, so I asume the max.pressure for calculating gets higher. This makes the form a little less secure, but because you fill in the originaladvice-pressures, in which this factor is included,it should give a higher ax-load on the rear, and so it is taken into account inonly a little less secure way.

Yes,you are wonderful.but I am not very clearly.how to caculate the pressure step by step. there is a example,could you help me to caculate it?
the weight of the car=1305kg
fornt axle=784kg
rear axle=521kg
the Vmax=205km/h
the power of the engine=96kW
the tire is 195/65 R15

How about the pressure of the tires? I think should caculate according the ETRTO manual. somebody told me some fomules how VW to do it. but I don't understand it clearly because I have no ETRTO manual.
danke!

The load index is verry probably 91 = 615kg per tire maximum load.
front axle 784kg empty with 2 persons of 75kg is + 75kg to the front is 859kg/0,85=1010kg >> 2,5* ( 1010/1230)^1,25= 1,953 rounded up to 2 bar op to 160km/h. Because you most probably have tires with speed code V , the 2,5 becomes 2,8
so the pressure above that speed of 160 is 2,2 bar
Rear Ax 521+75=596/0,85=701kg 2,5( 701/1230)^1,25=1,3 rounded up so use 1,4 because its the minimum pressure to use, which is also the pressure for above 160km/h up to 205km/h.
This is for normal use or when you are alone in the car. For heavy duty I need the maximum axis-loads of front and rear.
I am asuming here that you have normal car tires , an no reinforced or Cargo-tires.
Found out in the time I didn't react here that you must not use the maximum pressure of the tire but the reference-pressure. That is with normal tires 2,5 bar and with reinforced tires 2,9 bar . For C tires you use the the pressure on the sidewall of the tire( At65psi or At450kPa). Also made a new form for recalculating the advice-pressures on the stikker on the car when you place other tires then original, and placed it on my Sky-drive at Hotmail. http://cid-a526e0eee092e6dc.skydrive...uk%20omrekenen In this map always use the last one, and sorry, it is in Dutch.
This form replaces the other ones, and workes exactly the same as they used for the stikker. Begame wiser in time.