You should ALWAYS go by what the placard on the car says. The numbers on the tire mean that this tire can be run at that maximum pressure and not fail.
If people run their tires at too high of a pressure the surface of the tire touching the road is too small to provide good traction. Too high of a tire pressure can also cause the tire to overheat and the rubber will break down resulting in a blow-out while being driven.
Tires should be filled cold and if you can find a tire shop that fills tires using nitrogen that is the cutting edge technology. Nitrogen does not expand and contract with the tire temperature like regular air pulled into a compressor, so what you set it at is what it stays at unless you have a leak.
Good luck,
Kaiser Willy
