GMC 2006 Sierra - Manual (Page 381)

Operating the vehicle in Tow/Haul when lightly loaded
or with no trailer at all will not cause damage. However,
there is no benefit to the selection of Tow/Haul when the
vehicle is unloaded. Such a selection when unloaded
may result in unpleasant engine and transmission driving
characteristics and reduced fuel economy. Tow/Haul is
recommended only when pulling a heavy trailer or a large
or heavy load.
Weight of the Trailer
How heavy can a trailer safely be?
It depends on how you plan to use your rig. For example,
speed, altitude, road grades, outside temperature and
how much your vehicle is used to pull a trailer are all
important. It can also depend on any special equipment
that you have on your vehicle, and the amount of tongue
weight the vehicle can carry. See “Weight of the Trailer
Tongue” later in this section for more information.
Maximum trailer weight is calculated assuming only the
driver is in the tow vehicle and it has all the required
trailering equipment. The weight of additional optional
equipment, passengers and cargo in the tow vehicle
must be subtracted from the maximum trailer weight.
Above the 5,000 lb (2 268 kg) trailer rating, the
handling/trailering suspension is required on
C-1500 models and the handling/trailering or
off-road suspension is required on K-1500 models.
Use one of the following charts to determine how much
your vehicle can weigh, based upon your vehicle model
and options.
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