Resistor increases Ohms
0 ohms is no resistance, a direct connection
Infinite ohms is no connection
Ford fuel gauges use the Ground side as the control
The gauge is powered with 5volts, pulsed usually, and its needle shows the resistance to ground thru the sender in the tank
So in the case of a pre-1989 fuel level system a short to ground would be 0 ohms(under 10 ohms) so shows Full on gauge
A disconnected wire is infinite ohms(higher than 80 ohms) so would show Empty
Yes, the specific Empty ohms is 78, I just use 80 because its easier to remember, lol, and the difference is not a big deal in this type of system
The sender in the tank is just a coil of wire that is grounded at one end, the coil has a resistance of 10 ohms at grounded end and 80 ohms at the far end
The wire that goes to the gas gauge is attached to a metal arm inside the sender and to the float
As you fill the tank the metal arm is moved closer to the end of the coil of wire that has the ground, until gauge sees 10 ohms to ground
As the fuel level drops the metal arm moves farther away from grounded end so resistance thru the coil of wire gets higher
Image here:
http://www.fordification.com/tech/wiring/fsu08.jpg
IF...........there is any corrosion inside the sender, on the coil, that will increase the resistance to ground when the arm moves to that area of the coil, so gauge drops lower than it should
These can be opened and cleaned, and tested with ohm meter, but would involve pulling the pump assembly
And it could be the gauge itself
You could get 80, 60, 40 ohm(approx.) resistors to test if gauge is showing correct needle movement at the lower end, and then stability with fixed resistance
In 1983 the fuel gauge to sender wire, should be an Orange wire