just bought a 68 cutlass conv.
I was told that the gauge worked, but was off. when I first drove the cat, it showed about 1/2 tank. I put 5 gallons in it and it when to full. after driving it around the gauge dropped to 7/8 and never went lower. I finally stopped to fill it up and now the gauge reads way past full (into to hot idiot light in the same pod) and does not seem to move. any ideas where to start to trouble shoot this?
I looked here under the general info pages, http://my.core.com/~faldrich/fuelgage.htm
but was wonder if someone could tell be the symptoms if it sounds like the gauge, send unit, wiring or ground?

Believe that car only has two wires attached to the gage. 1, 12 volts. 2, Sender from the tank. A lack of ground to the meter can cause a super-full reading. If that meter doesn't have 3 wires, it gets it's ground from the meter frame.
If so, try using a test jumper lead and ground the meter frame to a good clean dash frame ground. With the key ON, see if the meter moves to normal.