All Fiero and many other GM lines... None of the Gauges are "Zeroed" when car is shut down for any reason.
And yes, changes in magnetic fields can make them move w/o power. How much depend on strength and orientation of magnet(s) etc.
Gauges Should read Zero or whatever when car is turned on. IOW Fuel gauge and others are only accurate when key is on.
GM might have covered this in the owners manual... get @
http://www.fieronews.netGM did publish a TSB covering this "problem"/complaint by owners but most don't have access to read them.
Even then don't trust most or all of them because iffy wiring etc can make them lie.
Worse on any 15+ year old car not just 30+ year old Fiero. Sender, gauge or anything else can make the gauge lie. Even just One iffy ground in the car can make make any or all gauges read off.
Example: Low or High Oil Pressure on the dash gauge often can't be trusted until you check w/ a real gauge on the engine.
If real gauges = same as dash gauge then you have problems that can wreck the engine.
V6, VIN-9, 30-45 PSIG
L4, VIN-R, 36-41 PSIG
Low pressure is "common" problem for V6
but Is Not Normal regardless of posted here and other places and engine can die in minute to years depending on many factors.
Many dash gauges read high even after replacing sender and even the gauge because of iffy wiring adds to resistance from the sender. (Fuel/OP senders operates only 0-90 Ω.) But Real high pressure when tested can be bad for the engine too. Oil Pump for most engines are driven by the cam or timing set and high pressure puts a huge load on cam and timing set.
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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)
The Ogre's Fiero Cave