While MagicRat got the right answer, unfortunately he got some of the details wrong.
First, what is written on the sidewall of the tire is typically a MAXIMUM. It will say something like "Max Load XXXX, Max Pressure YY". However it might say "Max Load XXXX at YY pressure" This indicates a relationship, but it doesn't mean that this is what should be used when the vehicle is fully loaded - it's an indication of the realtionship for the TIRE!
Further, what is listed on the vehicle tire placard (the sticker mentioned) is for a fully loaded vehicle (unless it states otherwise). If the placard doesn't have any exceptions or explanations, the pressure listed there should be used as stated.