GMC Sierra Denali Classic 2007 Owner's Manual page 106

REVERSE (R): Use this gear to back up.
Notice: Shifting to REVERSE (R) while your
vehicle is moving forward could damage
the transmission. The repairs would not be
covered by your warranty. Shift to
REVERSE (R) only after your vehicle is
stopped.
To rock your vehicle back and forth to get out of
snow, ice or sand without damaging your
transmission, see If Your Vehicle is Stuck in Sand,
Mud, Ice, or Snow on page 294.
NEUTRAL (N): In this position, your engine does
not connect with the wheels. To restart when
you are already moving, use NEUTRAL (N) only.
106
CAUTION:
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Shifting into a drive gear while your engine
is running at high speed is dangerous.
Unless your foot is firmly on the brake
pedal, your vehicle could move very
rapidly. You could lose control and hit
people or objects. Do not shift into a
drive gear while your engine is running at
high speed.
Notice: Shifting out of PARK (P) or
NEUTRAL (N) with the engine running at high
speed may damage the transmission. The
repairs would not be covered by your warranty.
Be sure the engine is not running at high
speed when shifting your vehicle.