Observation using cruise while towing

Sonders · July 10, 2023

Curious if anyone else has same symptons as I've experienced with cruise control while towing. 22' 2500HD Custom gasser, towing a 28' bumper pull camper weighing in around 8500lbs. Ok, I've noticed every time I'm towing and have cruise control set, any slight increase in slope and the truck drops down 5 mph before truck decides to downshift and rpms shoot up to 4000 rpm, then it will slowly get back up to set cruise. Usually cruising at 55 (55 mph speed limit areas), 65 (65 mph speed limit areas) or 68 (70 mph speed limit areas). Doesn't matter which speed cruise is set at, it always will drop down 5 mph before it attempts to regain speed. I've gotten to the point where anytime I see any sort of hill coming up, I go ahead and give it some gas so it will stay on speed. I live in hilly Southern Indiana, so it gets real old real quick having to constantly get on the throttle to maintain speed. Is this something that cruise parameters can be adjusted somehow by dealer maybe? Only complaint I have with my truck so far.

newdude · July 10, 2023

Sounds normal to me and that you "might" be expecting a bit too much from cruise while towing? You can see the hills therefore you can plan ahead and add power before starting the hill. The truck can't see that. All it sees is an increase in load from the increase in incline and then once it sees that, it downshifts. It can't pre-downshift to reset the powerband/gear to then pull the hill.

No adjustments can be made other than human input aka you driving the truck at those instances. For example, set the shift lever to "L" and tap the "+" till it reads "L6". The truck will use gears 1-6 just like in drive. When you see a hill, you can then manually tap the "-" on the shifter to downshift the truck to a better gear for the hills. Its likely in "L6" with tow/haul on it will never it the highest gear just like in D. The gas/6 speed combo for example will probably just stay in 5th gear, so a downshift will put it in 4th or 3rd gear in D and you can achieve those with manual mode as well but before the hill.

Edited July 10, 2023 by newdude

Naughty Donkey Performance · July 10, 2023

Just checking if this is in tow/haul mode?

Sonders · July 10, 2023

yes, sorry forgot to add that, using Tow/Haul Mode. NewDude, I like your suggestion of shifting to L and using the +/-. I'm going to go that route on our next trip coming up in a few weeks. I guess maybe I should try going without tow/haul mode as well? I've read mixed reviews about using tow/haul.

ShotgunZ71 · July 11, 2023

What I used to do in those situations is lock out 6th and maybe 5th. It keeps the RPM up and you won't lose speed as bad, if at all. Also, I found that the towing MPG was actually better when doing that. The engine stayed in it's sweet spot and wasn't playing catch up constantly.

Sonders · July 11, 2023

great tip ShotgunZ71, appreciate it.

Jettech1 · July 13, 2023

While towing last week and last weekend I noticed 4mph was the max decrease in speed with the cruise set on 70 or 65. I did notice at -2 it would downshift into 5th and most of the time that took care of the grade. If it went to -4 it would downshift into 4th and that always took care of the grade and brought me back up to the set speed. This is with tow/haul mode off towing our 8k lb camper in 95' weather and of course LOTS of humidity...lol..

Jettech1 · July 14, 2023

@Sonders I'm wondering if there was an ECU update or change. My build date is 7/22. What is yours?

Sonders · July 14, 2023

5 hours ago, Jettech1 said:

@Sonders I'm wondering if there was an ECU update or change. My build date is 7/22. What is yours?

Mine was built 9/21, so an early 22' model.

Jettech1 · July 14, 2023

6 hours ago, Sonders said:

Mine was built 9/21, so an early 22' model.

If I was a betting man, of which I am in Vegas...lol....I would have to say that GM did some updates on the ECU in that time frame between yours and mine. Have you asked your dealer if there are any updates for the ECU?

64BAwagon · July 15, 2023

My understanding was that there were some changes between the 2021 and the 2022 gasser/ 6 spd , namely the 22 would lock out 6th gear in tow/haul. My 21 was happiest in 4th gear running around 63 MPH with 2050 RPM's. This i with a 12k fifth wheel.

64BAwagon · July 16, 2023

I noticed on my 2021 Gas/6 spd that the drop and rise of the speed during cruise use seemed a little more generous before a downshift. Meaning that the system would allow an almost 5 MPH drop when encountering a grade before it would downshift to regain the speed. This was fine with me because I felt it prevented a lot of shifting for some minor grades that were short. If I noticed It shifting a lot I would just shut the cruise down and control the throttle myself.

I do know that my 05 Duramax was less prone to downshifting due to the torque that it had. This was one of the advantages of the diesel and should be part of anyone decision between the gas and the diesel. All in all I think the diesel has the obvious advantage in this aspect.

Jettech1 · July 16, 2023

8 hours ago, 64BAwagon said:

I noticed on my 2021 Gas/6 spd that the drop and rise of the speed during cruise use seemed a little more generous before a downshift. Meaning that the system would allow an almost 5 MPH drop when encountering a grade before it would downshift to regain the speed. This was fine with me because I felt it prevented a lot of shifting for some minor grades that were short. If I noticed It shifting a lot I would just shut the cruise down and control the throttle myself.

I do know that my 05 Duramax was less prone to downshifting due to the torque that it had. This was one of the advantages of the diesel and should be part of anyone decision between the gas and the diesel. All in all I think the diesel has the obvious advantage in this aspect.

The diesel will always win when towing. I don't think anyone will ever argue that point. I have diesel envy, but at the cost and maintenance and the future down times for problems....yeah, I'll stick with my gasser for now. Now if GM solved all of those worries with the updated 2024 models.....I might be able to justify that extra costs, even though I don't need it. Time will tell with their updates. If they are stone cold reliable with no problems at all....I'm in!!! I want one so bad I can taste it...LOL...

UWSkier · July 17, 2023

This sure isn't the case with the 2024 gasser and 10 speed. The thing will downshift aggressively with a drop of 1-2 MPH of cruising speed. I'm hoping the transmission is just learning, but it seems to me like it will always grab one more downshift than is necessary to maintain momentum going uphill, like it thinks my trailer is heavier than it is. There were some uphill pulls on my recent drive where about 3400 RPM would have done the job while only losing perhaps 1-2 MPH, but it decided it wanted to use 4200 RPM to do it, accelerate to set speed, then shift up again, while still climbing. Manual shifting helped somewhat, but not always.

Edited July 17, 2023 by UWSkier

Jettech1 · July 17, 2023

24 minutes ago, UWSkier said:

This sure isn't the case with the 2024 gasser and 10 speed. The thing will downshift aggressively with a drop of 1-2 MPH of cruising speed. I'm hoping the transmission is just learning, but it seems to me like it will always grab one more downshift than is necessary to maintain momentum going uphill, like it thinks my trailer is heavier than it is. There were some uphill pulls on my recent drive where about 3400 RPM would have done the job while only losing perhaps 1-2 MPH, but it decided it wanted to use 4200 RPM to do it, accelerate to set speed, then shift up again, while still climbing. Manual shifting helped somewhat, but not always.

Interesting. Perhaps an 8 speed would have closed the gap better. Either way it is what it is. I'm still a huge fan of my 6 speed but I do fully realize the 10 speed would be better on hills while towing keeping the engine in it's power band.