GMC 1994 Yukon Owner's Manual page 81

Shiff 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 let you. 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. Ignore the SHIFT light when
you downshift.
Four-Wheel-Drive Vehicles Only: 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-LOW.
Manual Transmissions- Shift Speeds
If your speed drops below 20 mph (32 km/h), or if the engine is not running
smoothly, you should downshift to the next lower gear. You may have to
downshift two or more gears to keep the engine running smoothly or for
good performance.
A
CAUTION:
If you skip more than one gear when you downshift, you could
lose control of your vehicle. And you could injure yourself or
others. Don't shift down more than one gear when you
downshift.
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