Fuel Gauge Sending Units compared
Here's why the Spectra unit doesn't work right. In the photos below, I've opened up the box containing the sliding contact and the resistive wire. The variable resistance comes from the total length of wire between the sliding contact (that moves with the float) and the case, which is electrical ground. Each time the contact touches a new turn of the wire, an additional length of wire is added to the circuit and so the resistance increases. You can see that all turns of the resistive wire are of equal length, so as the sliding contact moves along, the resistance between the contact and ground increases by the same amount each time a new turn is contacted. Also, the turns are equally spaced, so every inch of contact movement adds the same number of turns to the resistor.
Here's why the Spectra unit doesn't work right. In the photos below, I've opened up the box containing the sliding contact and the resistive wire. The variable resistance comes from the total length of wire between the sliding contact (that moves with the float) and the case, which is electrical ground. Each time the contact touches a new turn of the wire, an additional length of wire is added to the circuit and so the resistance increases. You can see that all turns of the resistive wire are of equal length, so as the sliding contact moves along, the resistance between the contact and ground increases by the same amount each time a new turn is contacted. Also, the turns are equally spaced, so every inch of contact movement adds the same number of turns to the resistor.