Shifting from Drive to Low

[Another JR]

I have a 2021 3500HD SLT Gas. I’ve read the manual, which says you can shift into L at any speed, and then use the + and - switch to set the highest allowed gear within limits set by the transmission computer. 
 

When I’ve tried it at low speed, the transmission downshifts to the lowest possible gear for the driving speed. Is that what happens if I was to shift into L at 60 or 70 when starting a climb?  Shifting down to 3rd or 4th and 4 or 5k rpm, or does it stay in the current gear until you set a lower highest gear? I’ve been too cautious to try it. 

[mrjulian416]

At higher speeds it stays in the gear it's in when you shift to L.  You can then drop that gear by hitting the "-" button, but only if the computer agrees you will not over rev by doing so.

[newdude]

Shifting to L from 5th or 6th gear will drop it to 4th.  Has been this way for quite some time, at least 2015 on the HDs.  

[Another JR]

So those two answers above provide conflicting information.  Do you know which is correct for a 2021?  I’ll ask the dealer, but I’m not sure if I can trust their answer given their track record on technical questions. 

[Grjfer]

1 hour ago, Another JR said:

So those two answers above provide conflicting information.  Do you know which is correct for a 2021?  I’ll ask the dealer, but I’m not sure if I can trust their answer given their track record on technical questions. 

@newdudeis correct. I use L often during my daily driving just see if it makes any difference in milage. Such as allowing the truck to coast rather than downshifting. At normal speeds 50-60 mine always shifts to 4th. Lower speeds it usually drop 1 gear below the current gear in use. 

[newdude]

3 hours ago, Another JR said:

So those two answers above provide conflicting information.  Do you know which is correct for a 2021?  I’ll ask the dealer, but I’m not sure if I can trust their answer given their track record on technical questions. 

 

Here is the chart that the dealer would pull up.  They used to put this in the owners manual but not sure if they still do.  2014 1/2 ton trucks was the first time GM did it this way, followed by the HD in 2015 and they've been that way since.  The 2007-2013 trucks if you went from D to M it wouldn't downshift.  It would indicate "M6" or whatever gear you were in.  

 

 

Quote

 

In vehicles with gasoline engines, when the shift lever is moved from D (Drive) to L (Manual Mode), a downshift may occur. The gear that the transmission is operating in when the shift lever is moved from D (Drive) to L (Manual Mode) determines if a downshift occurs. See the following charts.

6-Speed Automatic Transmission

Gear before shifting from D (Drive) to L (Manual Mode)	6th	5th	4th	3rd	2nd	1st
Range after shifting from D (Drive) to L (Manual Mode)	L4	L4	L3	L2	L2	L1

8-Speed Automatic Transmission

Gear before shifting from D (Drive) to L (Manual Mode)	8th	7th	6th	5th	4th	3rd	2nd	1st
Range after shifting from D (Drive) to L (Manual Mode) – Tow/Haul not engaged	L6	L6	L5	L4	L3	L3	L2	L1
Range after shifting from D (Drive) to L (Manual Mode) – Tow/Haul engaged	L6	L5	L4	L3	L3	L3	L2	L1

10-Speed Automatic Transmission

Gear before shifting from D (Drive) to L (Manual Mode)	10th	9th	8th	7th	6th	5th	4th	3rd	2nd	1st
Range after shifting from D (Drive) to L (Manual Mode) – Tow/Haul not engaged	L7	L7	L7	L6	L5	L4	L3	L3	L2	L1
Range after shifting from D (Drive) to L (Manual Mode) – Tow/Haul engaged	L7	L7	L6	L5	L4	L3	L3	L3	L2	L1

 

Edited August 19, 2021 by newdude

[Another JR]

Thanks everyone, and thanks newdude for posting that chart.  That answers my question clearly. 

[mrjulian416]

Well, I'll concede to what that chart says, but at 70mph it sure didn't rev like it dropped two gears.

[mrjulian416]

I stand corrected. Not sure what I was doing that last time I shifted to L, but just now it behaves like the table says.