I'm getting around 20 mpg on the highway in my 84 2.5L Fiero Sport. Im running a 180 thermostat with a 185 fan switch. Does anyone know if I need to advance my timing or reduce my spark plug gap to make up for running the engine colder? I've got brand new everything on this motor.

Fieros from the factory had a 195* thermostat. The computer was set up to run the most efficiently at that temperature. The fan switch was set at 235*. It seems hot, but again, the engineers were looking for complete combustion and the computer was set up for it. If you have an automatic transmission you should get at least 25 highway, and 30 if you have a manual transmission.

Your running cooler so the ecm is adjusting as needed and your fan is running more putting more electrical load on the engine.
If you want to test if this is the issue, unplug the fan switch so the fan does not run and put your 195 stat back in and see what you mileage does. If you need the fan on warmer days and you have AC you can pull the compressor fuse and turn on you ac to run the radiator fan if needed. If you try this and your mileage improves you have you answer.