GMC Sierra 2015 Owner's Manual page 304

In vehicles with gasoline engines,
when the shift lever is moved from
D (Drive) to M (Manual Mode), a
downshift may occur. The gear that
the transmission is operating in
when the shift lever is moved from
D (Drive) to M (Manual Mode)
determines if a downshift occurs.
See the following chart.
6 Speed Automatic Transmission
Gear before shifting from D
(Drive) to M (Manual Mode)
Range after shifting from D
(Drive) to M (Manual Mode)
8 Speed Automatic Transmission
Gear before shifting
from D (Drive) to M
(Manual Mode)
Range after shifting
from D (Drive) to
M (Manual Mode)
Tow/Haul not engaged
Range after shifting
from D (Drive) to
M (Manual Mode)
Tow/Haul engaged
Grade Braking is not available when
Range Selection Mode is active.
See Tow/Haul Mode on page 9-38.
Driving and Operating
6th 5th 4th
M4 M4 M3
8th 7th 6th 5th
M6 M6 M5 M4
M6 M5 M4 M3
While using Range Selection Mode,
cruise control and the Tow/Haul
Mode can be used.
9-37
3rd 2nd 1st
M2 M2 M1
4th 3rd 2nd 1st
M3 M3 M2 M1
M3 M3 M2 M1