Why does the fuel gauge needle drop more quickly below half-tank point?

Fuel gauges are notoriously inaccurate. Some of this is intentional on the manufacturers part, since running a fuel tank completely empty can damage modern fuel pumps which are expensive to replace. In other words the manufacturer expects the driver to refuel when the gauge reads empty, even though there may be 10 to 20 litres of gas still cooling the fuel pump.

The other major factor influencing fuel gauge inaccuracy is the design of the fuel level sending unit in the fuel tank and the shape of the tank itself. Fuel level sending units are simple devices which have not changed very much in the last 60 years. The fuel gauge receives electrical input from the sender unit which provides current through a variable resistor. The resistor is constructed in such a manner that the input of the resistor slides along a resistance element which is grounded on the fixed side. This variance in current is what determines where the level is indicated on the actual gauge. The inaccuracy comes from the mechanism that moves the resistor slider, which is a simple float mechanism attached to a metal rod. While the fuel level is dropping vertically from top to bottom of the tank, the float is actually moving in an arc with it’s central point attached to the pivot point of the float arm. There is always a difference between actual fluid level of the fuel and the movement of the resistor wiper because of angular movement of the arm through the arc.

If you consider the function of the fuel gauge from the manufacturers perspective, and most peoples actual use for a fuel gauge, accuracy is not a major consideration. While the perception of the fuel gauges needle movement is not affected by the drivers psychological interpretation, the meter itself provides a psychological comfort / anxiety scale for the driver. When a driver approaches the “Empty” mark on the gauge they usually experience some anxiety which motivates them to refuel, thereby avoiding damage to the fuel pump, injector system and possibly the engine under some circumstances. Consider that the manufacturer could just simply put a warning light in the dashboard, without any gauge at all, and achieve the same mechanical result i.e. a low fuel warning. This arrangement however would cause an elevated anxiety level for the driver who had no advanced warning from a monitored gauge.

The end result is that the fuel gauge is a safety feature to provide comforting input to the driver, with low fuel anxiety as a last resort to protect primarily the vehicle and secondarily the driver. It’s not designed to be accurate, it’s designed to give psychological input to prompt actions on the part of the driver.

source; Quora