Topic: Fuel and Temperature Gauge not reading correctly

[BoJo]

The fuel gauge goes to full then drops back to 3/4 of a tank.  I have replace the sending unit, tried 3 different voltage regulators as well as tested voltage and it is correct, put external grounds everywhere thinking it might be ground.  Any other suggestions.

[midlife]

That's normal if what you are talking about when you first power up the car.  The reason is that the constant voltage regulator on the back of the dash is a bimetallic strip that switches from 12V to 0V to average out 5V or so.  The switching is fairly slow, so its average right at the start isn't 5V.  If your gauges drop to normal values within about 15-30 seconds, I wouldn't worry about it.  Or...simply get an electronic version of the CVR.

[BoJo]

The problem is that the tank is full but only reading 3/4. 

[jwc66k]

I have replace the sending unit, tried 3 different voltage regulators as well as tested voltage and it is correct, put external grounds everywhere thinking it might be ground. 

What were your voltage readings, and where did you take them?
Disconnect the low voltage regulator leads (black-green stripe) to the temperature gauge and oil pressure gauge and try again. This eliminates those two items from the circuit.
You mention temperature gauge in your subject but make no mention of it in the text.
Measure the resistance of the fuel sending unit, 10 ohms full, 75 ohms empty (per Fred Ballard aka flyingfred). Yes, the tank must be drained and sender out to do this. The removal also allows a visual inspection of the resistance wire and wiper arm of the sender. They do wear out on 50 year old cars.
Jim