Ford Mustang Owner's Manual page 24

Tachometer
The tachometer (GT similar)
The tachometer displays the approximate engine
revolutions per minute (rpm), or how fast the
engine is running.
The white zone shows the highest number of
engine revolutions per minute (rpm) at which
you should drive your car. Driving with the
pointer in the red zone may cause engine
damage.
Battery Voltage Gauge
This gauge shows you the battery voltage when
the ignition key is in the ON position.
If the battery is operating under cold weather
conditions, the pointer may indicate in the upper
range of the NORMAL band while the battery is
charging. If you are running electrical accessories
with the engine off or idling at a low speed, or
the battery is not fully charged, the pointer may
move toward the lower end of the NORMAL
band.
If it stays outside the NORMAL band, have your
vehicle's electrical system checked as soon as it
is safely possible.
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