The way GM wired the directional lights for a long time results in a continuous indicator light in the dash if one of the turn signal lights fails. You could be looking at a burnt out bulb, or just a bad connection at the bulb. Turn on your four way flashers and walk around the car. If the bad light doesn't become obvious, turn on your parking lights and do it again.
The tach boards of our Fiero's have a resistor chip in them that goes bad, increasing resistance and causing the tach to read high. You can take the tach apart alter the board and add a variable resistor that allows you to adjust the tach's output.
http://www.fieros.de/en/articles/tach.htmlOtherwise, you can also purchase a new board to replace your existing tach board. I sell new replacement boards that allow for adjustment of the tach. Included on the board is a calibration circuit that allows you to accurately set that adjustable resistor.
$49 for an adjustable tach
$79 for an adjustable tach that includes the calibration circuit.
$89 for an adjustable tach that includes the calibration circuit AND a low fuel indicator light.
If you have an 84 tach with the voltmeter as part of the tach, only the second two options apply. I am currently dealing with an error with the voltmeter tach option and shipping will be temporarily delayed if you need it. No problems if you have an 85+ tach that has the oil pressure gauge, or the GT tach that does not have any additional gauge on the tach face.
http://www.reddevilriver.com/Tachometers.html