Hello, I also have the same issue as Steve, on my '78 Avanti II. I Fill the tank and it only reads a bit over 1/2 or 3/4 but never full, gauge flutters radically when driving. Is there 2 problems or just one ? Gunslinger..the fuel sending units on both your Avanti's cured what issue ? The fuel gauge not reading the correct amount of fuel in the tank or the fluttering gas gauge , or both ?
I will try the method Mr. Wood is suggesting to determine which one, gauge or sending unit, is at fault. It seems to me that there are 2 separate issues here, 1- gas gauge not reading fuel level accurat