The fuel gauge measures the amount of fuel in the fuel tank. The vehicle operator reads the fuel gauge from left to right. When the gauge is on the far left the tank is near empty, and when the gauge is on the far right the tank is near full. A gauge in the middle means that the fuel tank is half full. The owner's manual states the tank size.
A fuel tank must be read as a conservative estimate of how much fuel is left inside the engine. A fuel gauge can sometimes display a lower fuel level than what is actually available inside the fuel tank, thus making the reading inaccurate at times.
In an analogue fuel gauge, the level of fuel is measured by a sending unit inside the fuel tank. This unit made using a resistor that connects to a float through a thin metal rod. This system measures the level of the fuel tank by measuring how much current is sent through the float. The float generates an increasing amount of current as it sinks lower into the fuel tank. This mechanism is a reason for fuel gauge inaccuracy as the float does not begin to drop until the fuel level in the tank has dropped significantly.
As of 2014, new car models uses a microprocessor that reads a fuel tank's variable resistor which then sends the data to an electrical display on the car dashboard. The fuel gauge reacts slower to changes in the fuel level using this system as an average of the resistor movement is calculated by a built-in software as set data points.