1985 pontiac fiero gt, my speedometer, odometer do not work and my fuel gauge is messed up, when it's at half a tank it's empty?

Sounds like 3 separate issues (but maybe the speedo and odo are related). Lets start with the fuel gauge. I believe the sending unit is inside the tank and is most likely the cause but it could also be the gauge itself. The gauge responds to a voltage signal from the sender so it can be tested by applying different voltages to it. A repair manual will likely tell you what the readings should be. If it responds correctly, replace the sending unit otherwise, replace the gauge. As for the ODO, its internal, plastic gears are probably shot. If you can get a replacement gauge at a junkyard, the gears can be replaced or you may want to just replace the whole gauge and reset the milage by adjusting the gear settings. The SPEEDO could be several things but start with the sending unit somewhere in the drivetrain - I am not sure on a Fiero being it is a mid engine car, but check for a sending unit. That, when working, will translate through an electrical circuit to be read by the gauge. Any of these could be the problem and this circuit may also be driving the ODO operation as well. Good Luck.