On many cars, turning on the AC puts a load on them, causing them to run hot. In the case of the Fiero, it helps to keep it cool. The engine, that is.
When the temperature control buttons are in the Max, Norm, or Def position, the cooling fan is energized. We often unplug the cycle switch on the receiver-dryer when we're running in the hills. Climbing mountains and constantly running in 3rd gear raises the coolant temp, so we turn on the AC and they don't get a chance to get hot.
Try the AC trick before you get into bumper to bumper traffic. Keep it cool rather than trying to cool it down. Probably no need to go to the lower temp thermostat as that can sometimes cause it to run too cool, telling the ECM to dump more fuel into the system.