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Been driving my 67 for about 2 1/2 years now. One day I ran out of gas and my stock gauge read 1/4 tank. So I put about 10 gallons in and the gauge needle pegs FULL. I know it isn't full. So now I don't let it go under a 1/4 tank.

Any ideas why suddenly the gauge goes wacko on me? It is an aftermarket gauge I installed a couple 2.5 years ago. Do they just go bad? I also had the cluster gone through then too. I would understand if it showed less fuel if the float had a pin hole.

Its always something ya know?!
 

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Could be the gauge, could be the sending unit in the tank. The way they work is the gauge is actually reading voltage. The float in the tank has a potentiometer on it (like a 'volume control') that is a resistor that changes resistance as the float moves up and down. "Full" is one voltage and "empty" is a different voltage (I forget which one is "high" and which one is "low" - but you get the idea). One thing you can do that's at least a partial test on your gauge, is disconnect the wire at the tank (usually they connect near the front middle of the tank) and try it with the ignition on. With the wire disconnected the gauge should read one exteme (either completely full or completely empty) and if you short it to ground it should read the 'other' extreme. If that doesn't happen, suspect the gauge. If it does, then suspect the sending unit inside the tank.

Bear
 

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In addition to what BearGFR said, check the wire connection to ground at the sending unit, if there is a loose, corroded or bad connection this will cause resistance in the wire and will cause a false reading at the fuel gage. The way the fuel gage works is it receives a steady positive 12 volts at the gauge and the ground for the gage goes thru the sending unit.
 
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