GMC 1999 Sierra 1500 Pickup Owner's Manual page 238

Your Driving and the Road
drivers behind you are seeing your signal when they are not. It's important to
check occasionally to be sure the trailer bulbs are still working.
Driving on Grades
Reduce speed and shift to a lower gear before you start down long
or steep
downgrade. If you don't shift down, you might have to
use your brakes so
much that they would get hot and no longer work well.
On a long uphill grade, shift down and reduce your speed to 45 mph (70
kmlh) or less to reduce the possibility of engine and transmission overheating.
IT
you have an automatic transmission, you should use D (or, as you need to,
a lower gear) when towing a trailer. Operating your vehicle in D when towing
a trailer will minimize heat buildup and extend the life of your transmission.
Or, if you have a manual transmission with fifth gear and you are towing a
trailer, it's better not to use fifth gear. Just drive in fourth gear (or, as you
need to, a lower gear).
Parking on Hills
You really should not park your vehicle, with a trailer attached, on a hill.
If
something goes wrong, your rig could start to move. People can be injured
and both your vehicle and the trailer can be damaged.
But if you ever have to park your rig on a hill, here's how to do
it:
1. Apply your regular brakes, but don't shift into P (Park) yet, or into gear
2. Have someone place chocks under the trailer wheels.
for a manual transmission.
3. When the wheel chocks are in place, release the regular brakes until the
chocks absorb the load.
4. Reapply the regular brakes. Then apply your parking brake, and then
5. If you have a four-wheel-drive vehicle, be sure the transfer case is in a
6. Release the regular brakes.
shift to P (Park), or R (Reverse) for a manual transmission.
drive gear-not in N (Neutral).
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