Fuel Gauge/sender

[RumbleBee]

My fuel gauge has never really worked. The needle moves from 3/4 full (when the tank is empty) to beyond full when the tank is full. I have taken the sender out and tried bending the arm but this didnt help. It seems that the sender is working ok as when I move the float to empty, the gauge shows 3/4 full and when I move the float right up, the guage goes past full. It appears as if the guage needs to be calibrated. Will putting a high value resistor in the green wire from the sender help? There is also a black wire to the sender. Also coming out of the loom at the same point but not connected to anything is a red wire and a blue wire. Anyone know what these are for?

Many Thanks

B