The gauge and low-fuel light use different senders. The sender for the low-fuel warning is either malfunctioning or there is a wiring fault.
The sender is in the tank and something is wrong with it. It probably needs a new sending unit.
your float is stuck or you have a short in the hot wire!
I drive from Inverness to Telford 450miles on a full tank with the light just coming on.
might be a bad sending unit
The sending unit in the fuel tank is defective,, replace it and fix your problem.
This is usually cause by a vacuum leak in the fuel tank evaporative system.
Defective sensor
If your 2005 Nissan Frontier has a full tank but the fuel indicator shows empty and the service engine light is on, you may need the computer or the "brain" reset or replaced. You can take the truck to a garage or to the Nissan dealer to have this done.
Fuel level sensor malfunction.
I'm guessing you mean when the tank is full. It is probably the fuel sensor.
Sounds like sending unit in tank is stuck or elec. problem with it.