Fuel gauge not working. the tank is always "empty"

[varg300]

The gauge has never worked as long i have had the car.
Even if i fill the tank completly full, the gauge still says that the tank is empty. I have just reset the trip-meter every time i have filled, but this is starting to get annoying. Where is it most likly that the problem is? The sensor, or the gauge itself?
And one more thing, i have just attached a tacho, but what is the idle rpm for the engine?

[burn_your_money]

Idle should be around 700-1000. I set it to wherever it shakes the least.

I'd start by looking under the rear seat with a ohm meter on the fuel gauge sender. You could actually take it out and move it and you should get varying resistance. If that is good I would suspect the wires and then the cluster.