Ford Mustang Owner's Manual page 76

Speed Control (If equipped)
The speed control switches on the steering wheel
To Turn Speed Control Off
Press OFF, OR
Turn off the vehicle ignition.
Once speed control is switched off, the
previously programmed set speed will be erased.
To Turn Speed Control On
Press ON
To Set a Speed
Press SET ACCEL. For speed control to operate,
the speed control must be ON and the vehicle
speed must be greater than 30 mph (48 km/h).
If you drive up or down a steep hill, your
vehicle speed may vary momentarily slower or
faster than the set speed. This is normal.
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