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I noticed this recently with newer cars with electronic displays, in that for whatever reason, if you only add a gallon or two of gas, the fuel gauge won't update. However, if you fill it from a quarter tank to full, it will get adjusted accordingly.

Why do they act like this? I had assumed that they were measuring the volume filled of the tank itself, but then why would a gallon or two of gas not update at all?

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That is down to the resolution or accuracy of the sensors used.

To consider an “old” example, the sensor was a variable resistor with the float moving the wiper across the resistor. Had a customer who was not happy with the poor gauge reading - and so we re-wound the resistor so it was a better indication of the fuel level.

Some sensors are now a set of switches in a tube operated by a magnet - so cheap is paramount and the number of switches controls the resolution.

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