GMC 2001 Envoy - Manual (Page 168)

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Here are four things that some owners ask about.
None of these show a problem with your fuel gage:
D At the gas station, the gas pump shuts off before
the gage reads full.
D It takes a little more or less fuel to fill up than the
gage indicated. For example, the gage may have
indicated the tank was half full, but it actually took a
little more or less than half the tank’s capacity to fill
the tank.
D The gage moves a little when you turn a corner or
speed up.
D The gage doesn’t go back to empty when you turn
off the ignition.
Low Fuel Light
The LOW FUEL light will
come on briefly when you
are starting the engine.
This light comes on when the fuel tank is low on fuel.
To turn it off, add fuel to the fuel tank. See “Fuel” in
the Index.