GMC 2001 Sierra 1500 Pickup Owner's Manual page 120

You can choose among four driving settings:
2HI: This setting is used for driving in most street and
highway situations. Your front axle is not engaged in
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two wheel drive. This setting also provides the best
fuel economy.
AUTO 4WD: This setting is ideal for use when road
conditions are variable. When driving your vehicle in
AUTO 4WD, the front axle is engaged, but the vehicle's
power is sent to the rear wheels. When the vehicle
senses a loss of traction, the system will automatically
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engage four wheel drive. Driving in this mode results in
slightly lower fuel economy than 2HI.
4HI: Use 4HI when you need extra traction, such as on
snowy or icy roads or in most off
setting also engages your front axle to help drive your
vehicle. This is the best setting to use when plowing snow.
4LO: This setting also engages your front axle and
delivers extra torque. You may never need 4LO. It sends
maximum power to all four wheels. You might choose
4LO if you are driving off
deep snow and climbing or descending steep hills.
CAUTION:
Shifting the transfer case to NEUTRAL can
cause your vehicle to roll even if the transmission
is in PARK (P). You or someone else could be
seriously injured. Be sure to set the parking
brake before placing the transfer case in
NEUTRAL. See "Parking Brake" in the Index.
NEUTRAL: Shift the vehicle's transfer case to
NEUTRAL only when towing your vehicle. See
"Recreational Vehicle Towing" or "Towing Your
Vehicle" in the Index for more information.
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road situations. This
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road in deep sand, deep mud,
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