mines the same...put say a tenner in it and write down what ur mileage is at...then drive until u need to refuel (i.e. when the throttle gets light) then make a note of how far uv done...this way u can keep an eye on ur mileage/fuel ratios too...
Fuel gauge could be the voltage stabilizer ( 3 pronged with black sealed diode ) behind clocks or could be the circuit board.
Speedo could be your cable or try disconecting from the speedo and putting some grease round it.