Tim,
The markings on the tire are there for safety issues. They are independant of the car. A high PSI will distort the shape of the tire and lower the footprint area on the road, resulting in unsafe braking conditions.
The inflation figures on the car are there because of the manufacturer doing some testing of the tires on the car and determing the proper PSI for parameters such as drive comfort, braking distance, cornering and the like. It is a compromise between the various paramters based on how the manufacturer wants the car to perform with the stock tires.
I would suggest that these numbers are a good starting point for replacement tires. You may want to increase the PSI a bit depending on the tires you have, especially if you are carrying a lot of weight. I would not recommend a lower number since that may cause the tires to run hotter.
Tom
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