Chevrolet Low Cab Forward 2016 Manual page 210

If your vehicle is traveling at a
speed exceeding
135 km/h (84 MPH), it is not possible
to downshift from 6th gear to
5th gear.
If your vehicle is traveling at a
speed exceeding
104 km/h (65 MPH), it is not possible
to downshift from 5th gear to
4th gear.
Overdrive Off Switch : Under
normal circumstances, press the
overdrive off switch (3) to turn the
overdrive OFF (the overdrive off
indicator light comes on).
Press the switch again to release
the overdrive off (the overdrive off
indicator light goes off). See
Overdrive Off Light (Diesel
Only) 0 83.
Driving and Operating 209