I assume that your car was not originally equipped with P-metric tires and MB's inflation recommendation was based accordingly. I recently switched my P-metric tires on my Xterra to LT-tires, which are designed to operate under higher tire pressures. I contacted the tire manufacturer, told them what the original tire type and size was, what the new tire size and type is, and Nissan's air pressure recommendation as found on the driver's door.

They got back to me with recommended tire pressures, which I use. Apparently they look at the maximum weight that the original tire type is designed to support at the car manufacturer's recommended air pressure and determine what PSI the new tire has to be inflated to in order to support that same amount of weight.