> Any ideas how they're doing this?  Perhaps a servo?

It can be a stepper. Does it index (go to zero) sometimes ? Otherwise it
could be simply a differential instrument with deliberately high friction
bearings. With modern magnets that should be easy. When you say it works
w/o ignition, do you mean it shows what it showed when you switched off or
does it really show what's in the tank (assuming youare filling it).

Peter

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