GMC 2003 C4C042 Owner's Manual page 147

Engine Speed Limiting
All engines have electronic engine speed limiting.
The engine limited speeds range between 2,900 and
4,400 rpm depending on engine option content.
2-72
Engine Road Speed Governor
This system controls maximum vehicle speed
automatically and reduces engine power until vehicle
speed gets down to maximum governed speed.
A 75 mph (120 km/h) road speed governor is standard
for most models, but can be programmed to any speed
below that default setting. Other models have a default
setting of 80 to 85 mph (128 to 136 km/h). You may
have a label on the headliner of your vehicle with more
information on your engine road speed governor. See
your dealer for assistance with programming your
engine limited speed.