Are the shift points determined by speed or engine RPM?

[RVGuy]

For my own curiosity, I've been wondering this for quite a while now but haven't really found the info anywhere. There are some pretty smart people here so maybe one of you know?

Does the trans determine shift points by vehicle speed or by engine RPM? Say you're comparing two trucks, one with 3.21 gears and one with 3.92. Is the trans going to shift at the same vehicle speed on both trucks or are the shift points going to vary slightly due to the difference in engine RPMs at the same speed?

 

[Michael]

I’m not sure, but it might be a combination of the two.


I know the voices in my head aren’t real, but man, do they ever come up with some great ideas.

 

[crash68]

Aside from the rpms and speed, torque demand and engine load is used to determine when to shift up or down. There is probably times that transmission won't down shift going over an overpass, if your pulling a trailer you could be two gears down going over the same overpass at the same speed when unloaded.

 

[RVGuy]

Aside from the rpms and speed, torque demand and engine load is used to determine when to shift up or down. There is probably times that transmission won't down shift going over an overpass, if your pulling a trailer you could be two gears down going over the same overpass at the same speed when unloaded.

Yes, but assuming the exact same trucks with no additional load with the same driver.

I have 3.55 gears in my truck and I always get an upshift if I'm coasting at 60km/h which puts my RPMs at 1000. I was wondering if a 3.21 would shift at the same spot, forcing the RPMs below 1000 or if the computer would identify that the resulting RPM would be too low to effectively maintain speed and upshift later.

I know varying conditions and loads will vary shifts, I'm pretty much just wondering if the 3.21 gears would bog down the engine more than the others just putzing around town.

 

[crash68]

Yes, but assuming the exact same trucks with no additional load with the same driver.

I have 3.55 gears in my truck and I always get an upshift if I'm coasting at 60km/h which puts my RPMs at 1000. I was wondering if a 3.21 would shift at the same spot, forcing the RPMs below 1000 or if the computer would identify that the resulting RPM would be too low to effectively maintain speed and upshift later.

I know varying conditions and loads will vary shifts, I'm pretty much just wondering if the 3.21 gears would bog down the engine more than the others just putzing around town.

The transmission would upshift quicker with the 3.21 when coasting as the load has dropped off and as long as the engine will remain within it's upper and lower rpms limits. Downshifting would happen sooner due to increased load when accelerating. The shift points when there is not a load may happen at the same rpm but you'll be moving at a different speed(engine load will be different).
The 3.55 reduces the load to the engine by about 10% but all the truck suffer the same aerodynamic load of a brick.
If you have a Bluetooth OBD reader, you should be able to see the engine requested and actual demand (as the ECM calculates it) using the Torque App.

 

[RVGuy]

The transmission would upshift quicker with the 3.21 when coasting as the load has dropped off and as long as the engine will remain within it's upper and lower rpms limits. Downshifting would happen sooner due to increased load when accelerating. The shift points when there is not a load may happen at the same rpm but you'll be moving at a different speed(engine load will be different).
The 3.55 reduces the load to the engine by about 10% but all the truck suffer the same aerodynamic load of a brick.
If you have a Bluetooth OBD reader, you should be able to see the engine requested and actual demand (as the ECM calculates it) using the Torque App.

Thank you, this answered my question.

 

[ram1500rsm]

The trans shift points are based on transmission output shaft rpm vs Thottle position.

 

[bm02tj]

Throttle pressure and RPM with the ECU looking for best fuel economy as well

 

[kurek]

I remember reading the 8 speed has something like 40 distinct shift strategy maps based around demand, load, rpm, road speed, vehicle stability control data (are you climbing or just pushing against the wind?), velocity of throttle movement (did the driver mash the pedal or ease in?), temperature, time since starting..

Kinda cool tech as long as it works

 

[Wild one]

Wide open with your foot through the floor they all shift at roughly 5600 rpm,the 3.21's will be shifting at noticeably higher speeds then the 3.92's under those circumstances.

 

[Random_Walk]

Pretty sure there's an algorithm at work, and it combines quite a few factors mentioned above, plus a few others (O2 sensor stands out to measure effective altitude as an example), but consider that the Tow/Haul button changes all of that.

Consider further that the owner's manual specifically says that if you live in hilly/mountainous terrain, you'll want to basically leave your drivetrain in permanent Tow/Haul mode to keep your transmission happy and healthy, so I figure that Tow/HAul mode is likely a simpler, less shift-happy version of the regular algorithm?

 

[JohnnyMac]

I remember reading the 8 speed has something like 40 distinct shift strategy maps based around demand, load, rpm, road speed, vehicle stability control data (are you climbing or just pushing against the wind?), velocity of throttle movement (did the driver mash the pedal or ease in?), temperature, time since starting..

Kinda cool tech as long as it works

I believe it factors in whether a male or female is driving as well.

 

[Lyle Longboat]

lol

 

[Dave2018]

I believe it factors in whether a male or female is driving as well.

Well of course! That goes without saying.... Otherwise it's based on what kurek said...