Ford 2003 Mustang Owner's Manual page 15

Instrumentation
Fuel gauge
Displays approximately
how much fuel is in
the fuel tank (when
F
the key is in the ON
position). The fuel
E
gauge may vary slightly
when the vehicle is in
motion. The ignition
should be in the OFF
position while the vehicle is being refueled. When
the gauge first indicates empty, there is a small
amount of reserve fuel in the tank. When refueling
the vehicle from empty indication, the amount of
fuel that can be added will be less than the
advertised capacity due to the reserve fuel.
A minimum of six gallons must be added or removed
from the fuel tank in order for the gauge to
instantaneously update. If less than six gallons is the
change, the gauge will take between five to ten
minutes to update.
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