Don't bother looking for a data plate, it may be wrong.
This is especially true for older vehicles, where tire technology has surpassed what was available at the time of manufacture.
My brother ruined six tires on his motorhome following the recommendations of the data plate. The tires demanded more pressure- he ran them under-inflated and they shed their tread. No fun.
It may be that the pressure listed on the car is wrong for one reason or another, but the pressure on the tire is
never right.
If you really think that the listed pressure is wrong, you can always figure it out based on the shape of the tread. Find a smooth surface you can park on; smooth concrete would probably be ideal. Then get a feeler gauge. Overinflate your tires so that you can get the feeler gauge between the tire and the concrete on the side of the tire. Release the pressure until the feeler gauge no longer fits. You'll probably need to move the car a little bit after releasing the pressure to release friction between the tire and the ground. Record that pressure. Now release more pressure until the feeler gauge can slip between the tire and the concrete in the middle of the tread. (You might need to tape the feeler gauge to a stick to get it that far underneath the tire.) Mark that pressure. Now you have two pressures that you know to be too much and not enough. So set the pressure between them. Hopefully they will be close enough that you can draw some sort of conclusion.
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