GMC 2000 Sierra - Manual (Page 105)

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Shift Speeds
CAUTION:
If you skip a gear when you downshift, you could
lose control of your vehicle. You could injure
yourself or others. Don’t shift down more than
one gear at a time when you downshift.
If your engine speed drops below 600 RPM, or if the
engine is not running smoothly, you should downshift to
the next lower gear.
Shift Light
If you have a manual
transmission, you may have
a SHIFT light. This light
will show you when to shift
to the next higher gear for
best fuel economy.
When this light comes on, you can shift to the next
higher gear if weather, road and traffic conditions allow.
For the best fuel economy, accelerate slowly and shift
when the light comes on.
While you accelerate, it is normal for the light to go
on and off if you quickly change the position of
the accelerator.
If your vehicle has four - wheel drive and is equipped
with a manual transmission, disregard the SHIFT light
when the transfer case is in 4
- Wheel Low.
Ignore the SHIFT light when you downshift.