GMC 1996 Sierra Owner's Manual page 202

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If you have an automatic transmission, you can tow
in OVERDRIVE (@). You may want to shift the
transmission to THIRD (3) or, if necessary, a lower
gear selection if the transmission shifts too often
( e g , under heavy loads and/or hilly conditions). If
you have a manual transmission and you are towing
a trailer, it's better not to use FIFTH (5) gear. Just
drive in FOURTH (4) gear (or, as you need to, a
lower gear).
Three important considerations have to do with weight:
the weight
of
the trailer,
the weight
of
the trailer tongue
and the weight
on
your vehicle's tires.
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. And, it can also depend on any
special equipment that you have on your vehicle.
Use one of the following charts to determine how much
your vehicle can weigh, based upon your vehicle model
and options.
Maximum trailer weight is calculated assuming the
driver and one passenger are 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 2,000 lbs. (908 kg) trailer rating, the engine
oil cooler is required
on C/K- 1500
models with gas
engines and C/K-2500 models with gas engines and
3.73 (or lower) axle ratio.
Above the 4,000 lbs. (1 800 kg) trailer rating,
heavy-duty or gas shock absorbers are required
on C - 1500 models, and heavy-duty shock absorbers
or the off-road chassis package are required
on
K-1500 models.
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