Chevrolet 2010 Impala Owner's Manual page 182

When the ignition is on, the fuel gauge indicates about
how much fuel is left in the fuel tank.
4-36
An arrow on the fuel gauge indicates the side of the
vehicle the fuel door is on.
Here are four things that some owners ask about.
None of these show a problem with the fuel gauge:
At the gas station, the gas pump shuts off before
the gauge reads full.
It takes a little more or less fuel to fill up than the
gauge indicated. For example, the gauge may
have indicated the tank was half full, but it actually
took a little more or less than half the fuel tank's
capacity to fill the tank.
The gauge moves a little when while turning a
corner or speeding up.
The gauge does not go back to empty when the
ignition is turned off.