[jamis]

Can you be a little more specific.
What gauge are you using?

Mean while check the conductivity of the circuit with and ohm meter, since the gauge works reading resistance any resistance added other then the sending unit will affect your reading. With the wire off the sending unit, check the ohm reading at the gauge by placing one end of the meter on the terminal and the other on ground. Place back the wire on the sending unit, remove the wire that leads into the gauge and then check the ohm reading there. They should be very close.

There are many other things that can cause a bad reading such as component compatibility or a bad component. You may have to remove the sending unit and check its range of resistance. Also check that the sending unit is neither too short nor too long. the total length of the sending unit with the float all the way down should be a little short of the depth of the tank.

With the gauge out reconnect the wires and add a ground and check the readings at the gauge as you move the float threw its range. And tell us what its doing. If its a little off you can bend the float arm to get it closer.