I have just received the low fuel senders I had restored by Bill B. Desert Classic Parts. His service includes new brass float, new white filter, new thermister & hand calibrated!
These were my personal stash of senders I had collected over the decades.
All senders are $250 plus shipping & insurance. No cores required! ( although I would suggest sending your cores to Bill B.)
2 gallons will move the needle, and 4 gallons gets out of the red zone and turns off the low fuel light on my 70. That’s with a calibrated rebuilt sensor from Desert Classic Parts, and a 22 gallon tank. One thing to remember is that it can take a full minute or more with the ignition on for the gauge to respond, especially at the low end where there is minimal current to heat up the gauge coil.
Calicat has pretty well nailed it. Ford engineers mainly wanted you to not run out of gas.
Keep in mind the sender doesn’t measure gallons of gas. It measures distance above the bottom stop, about an inch above the bottom of the tank, and the top stop, also below the top of the tank. Fuel tanks are anything but linear in volume per inch of depth. when it says you have a half tank it means the level of the fuel is at the mid point of the tank, now necessarily 11 gallons. It is not a meter, it is a gauge. Ford also thought it essential to keep some reserve to accommodate parking on a slope and the prevent starvation during braking and going around corners.