GM-GMC: Wiring of a fuel gague for a 71 Chev Truck, chev pickup, chev truck

Question
Why does the pointer of a new fuel gauge for a 71 Chev Pickup move before it is even installed? Will it read empty when it is installed and all wired?

Answer
The gauge on my 84 K-2500 bounces around when driving the vehicle.  It does not accurately report the level of the contents of either tank, but it does let me know when a tank is nearly empty.

Fuel gauges are estimating devices only.  The sender is a variable resistor that varies voltage which the gauge translates into a dial reading.

I would not worry about the gauge moving before installation.  It should only read empty when the tank is empty, the sender is defective, or the float on the sender has a hole in it and it sinks to the bottom of the tank.