When the fuel tank is full the gauge is measuring it right but once it goes below 1/2 tank the needle
drops all the way to empty. The only way I know it still has fuel in it is by the mileage. I have people
tell me that I need to fill up when I have just driven it 100 miles.
Sounds like a bad sender, the sender is just like a rheostat with segments that tell the gauge how many ohms for different fuel levels, sometimes those segments get dirty or simply wear through and then the gauge looses the signal. Hope this helps.
Running around with less than half a tank for twenty years has worn a hole in the resister windings. Reset the trip odometer when you fill it up so you know how many miles you have gone.
I concur. It is your sending unit.
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