Because the gas in being forced to one side of the tank and the sensor that reads the fuel level is reading a lower level as gas gets sloshed around.
Most likely, there is a small hole in the plastic float in your gas tank's sending unit. As the float fills with fuel,it sinks to the bottom of the tank and sends an "empty" signal to the gas gauge. You're gonna need a new fuel pump. Scott
Looking from the rear of the gauge apply +12v to right terminal and ground to left terminal. If functional the gauge will go to full. Reverse wires and the gauge should go to the left below empty. If it does these two things the gauge is working and the problem is wiring or sending unit. Randy Nix
That depends entirely on the vehicle and on how accurate the gauge is.
The gauge or sensor needs to be replaced.
Reason to apply a gauge tolerance
Either the gauge is broken or the sender is broken.
Failed gauge, wiring, or level sensor.
I would assume that the wires are hooked up backwards
Your fuel sending unit is not giving the correct reading to your gauge
Where is it stuck? on full or empty? if on full you got a bad ground, if empty you got a bad float.
Bad gauge Bad float in tank
check your ground wire to the gauge