The gauges a slow to respond to change, so they effectively give you they average reading over 20-30 seconds. The gas might slosh about, and the little float go up and down, but it will stay centered on the average level (some tanks have little walls inside to make it slosh less). the only time you might see it change is when you are on a hill and the gas isn’t sloshing but settles towards one side of the tank – that will often cause the gauge to drift up or down.
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