25 rs awd 8 speed

[2025 Red Equinox]

It's a poor approach to vehicle performance, short term and long term. Short term, fuel economy has to be impacted just by the fact the engine is turning more RPM's at road speed. Slowing the engine while maintaining road speed will save fuel, place the engine RPM's in a more favorable position in the torque curve. The amount of fuel it saves is a moving target as there are a host of things that drive MPG besides RPM. Long term, engine life. Lowering the RPM thus reducing the number of revolutions in it's life time reducing wear. There has to be some reason GM has chosen to use this approach in their 8 and 9 spd transmission applications. Owners of vehicles (all GM but not just Chevrolet) say their 9 spd work the same way. It suggests the top gear is an overdrive. I don't have access to the total gearing to determine whether that's a fact or not. Even if it is, having it function in this manner doesn't make much sense. One other point to mention is how it performs in cruise control, especially when adaptive control is turned on. Once in 8th gear, cruise on the vehicle can perform very well. As a slower vehicle is approached the cruise will slow down to their speed. If they speed up quickly or pull off cruise will accelerate to the set speed but the rate will cause it to downshift to 7th. It will never return to 8th gear. To get back, ERS (paddle shift) must be used to get back into 8th. The default for ERS is 6th gear. When it's turned on engine RPM's will go to 3000 at 70 mph. Upshifting with the paddle will get to 7th (2200) then 8th (1800), if there's no acceleration load on the drivetrain. Seems there's too much interaction required to get the benefit of an 8 spd or 9 spd application.

I also have a 2024 Chevrolet Tahoe Premier 4WD with the 10-speed automatic transmission. I have heard others say that it will not often shift into the 10th gear. I cannot tell on my Tahoe if it reaches 10th gear on the expressway.

You’re right that there must be a reason GM is choosing to program their 8, 9, and 10-speed transmission's this way?

 

[PoManNox]

Best I can tell from online, the old 6spd 6T45's gear ratios range from 4.58 (1st gear) to 0.75 (6th gear)

The new 8spd, 8T45 is 4.69 to 0.62.

Not an earth shattering delta between the two from low end to high end, but the shorter steps to get to 0.62 is where the 8spd "should" gain some fuel economy.

I love the 6spd in my 2021 w/ 1.5T. It's perfect for 6 gears. IMO, it's not enough engine to squeeze two more gears into.

My guess here is GM programmed it to hit and hold 8th only in absolutely perfect conditions to save the unit from constantly flip/flopping in 8-7-8-7, on top the torque converter potentially having to go in/out of lockup in the process. People would complain about that as well.

On top of that, demanding higher torque at low RPM from these little 1.5T's isn't going to do them any good. Spark knock, LSPI, etc..

 

[Defflin]

It's a poor approach to vehicle performance, short term and long term. Short term, fuel economy has to be impacted just by the fact the engine is turning more RPM's at road speed. Slowing the engine while maintaining road speed will save fuel, place the engine RPM's in a more favorable position in the torque curve. The amount of fuel it saves is a moving target as there are a host of things that drive MPG besides RPM. Long term, engine life. Lowering the RPM thus reducing the number of revolutions in it's life time reducing wear. There has to be some reason GM has chosen to use this approach in their 8 and 9 spd transmission applications. Owners of vehicles (all GM but not just Chevrolet) say their 9 spd work the same way. It suggests the top gear is an overdrive. I don't have access to the total gearing to determine whether that's a fact or not. Even if it is, having it function in this manner doesn't make much sense. One other point to mention is how it performs in cruise control, especially when adaptive control is turned on. Once in 8th gear, cruise on the vehicle can perform very well. As a slower vehicle is approached the cruise will slow down to their speed. If they speed up quickly or pull off cruise will accelerate to the set speed but the rate will cause it to downshift to 7th. It will never return to 8th gear. To get back, ERS (paddle shift) must be used to get back into 8th. The default for ERS is 6th gear. When it's turned on engine RPM's will go to 3000 at 70 mph. Upshifting with the paddle will get to 7th (2200) then 8th (1800), if there's no acceleration load on the drivetrain. Seems there's too much interaction required to get the benefit of an 8 spd or 9 spd application.

Nicely worded. Agree 100 percent.

 

[GaryM]

Best I can tell from online, the old 6spd 6T45's gear ratios range from 4.58 (1st gear) to 0.75 (6th gear)

The new 8spd, 8T45 is 4.69 to 0.62.

Not an earth shattering delta between the two from low end to high end, but the shorter steps to get to 0.62 is where the 8spd "should" gain some fuel economy.

I love the 6spd in my 2021 w/ 1.5T. It's perfect for 6 gears. IMO, it's not enough engine to squeeze two more gears into.

My guess here is GM programmed it to hit and hold 8th only in absolutely perfect conditions to save the unit from constantly flip/flopping in 8-7-8-7, on top the torque converter potentially having to go in/out of lockup in the process. People would complain about that as well.

On top of that, demanding higher torque at low RPM from these little 1.5T's isn't going to do them any good. Spark knock, LSPI, etc..

On the surface you would think a 1.5 would cause a lot of shifting but my experience with my 2025 is it doesn't. It did surprise me. I had a 2.0T with a 6 spd prior and on one bridge it would always downshift, especially in cold weather. Thís one doesn't even work hard to do the same climb. I was concerned about "stepping down" to the 1.5 but it seems to do ok. Where it shows the most is inclined on ramps; the 2.0T did them much better. My take on this whole thing is GM has a reason we're not privy to for doing what they're doing. The dealers are not able to "educate" either because they don't know or are instructed not to. I just think if you have an "automatic transmission" it should be programmed to perform all the necessary functions to put the vehicle in the most efficient and effective mode for the speed and load it's working with. I'm not a lazy person but I don't feel we should be dealing with all the "stuff" we have to do just to get the most from our vehicle. Someone would have to show me a fuel map to convince me 7th gear at 2200 rpm is more efficient than 8th gear at 1800 rpm. Common logic says it's not.

 

[GaryM]

I found this;

 

[PoManNox]

I found this;View attachment 3292

The only problem with that is the 8L80 is a completely different rear wheel drive truck transmission.

 

[PeteComp]

So after reading this post I have been watching the tach and speed to determine what gear I'm in. Today, I put it into manual mode and was going about 70 at about 2200 RPM in 8th gear. I then put the cruise on to see if it would change but it didn't. However, the car slowed down because the person in front of me was going slower and it was on adaptive cruise. As soon as I moved to the next lane, it downshifted to accelerate (clearly showing the RPM's spike) but the shifter still showed 8th gear. Does the display only show what gear I set it to and not the actual gear I am in? Very strange.

 

[RIT333]

Yes, the numbers indicate the highest gear that your car can use, not the one it is using !

 

[GaryM]

I suspect that it is actually a 7-speed and they count the torque convertor lock-up as the 8th "speed".

Actually it's a 6 spd with two overdrives to give you 7th and 8th. The TCM is very sensitive when it comes to shifting to 7th and 8th and there are a host of things to consider. I found out the automatic trans fluid was not warm enough to allow the TCM to shift. Normal ATF on highway temp should be 175-215 deg F. Mine won't go above 140. The TCM threshold is 150 so it won't shift to 7th or 8th. It may go into 7th in light load (downhill and no throttle) and it may stay there for a while but eventually it will shift back to 6th. I'm working with the dealer to replace the thermal control valve for the transmission heat exchanger. If they stick closed the trans fluid never warms up. We'll see how it works after that.

 

[GaryM]

So after reading this post I have been watching the tach and speed to determine what gear I'm in. Today, I put it into manual mode and was going about 70 at about 2200 RPM in 8th gear. I then put the cruise on to see if it would change but it didn't. However, the car slowed down because the person in front of me was going slower and it was on adaptive cruise. As soon as I moved to the next lane, it downshifted to accelerate (clearly showing the RPM's spike) but the shifter still showed 8th gear. Does the display only show what gear I set it to and not the actual gear I am in? Very strange.

8th gear at 70 should give you 1800 rpm. The manual shift will show you've selected 8th but the TCM doesn't see the required criteria to actually shift to 8th gear so the trans stays in 7th gear, which is 2200 rpm. The dash will show 8th but only because you've selected it.