Curious why you say up to about 55mph? I am at the tail end of a 2 week family road trip and have been running in L9 on the highways averaging 75mph. Only seems to raise the RPM like 300 or so from just having it in Drive. I'm still getting like 420 miles to the tank (2024 RST PE).
As Marky Dissod stated, there is very little difference between 9th and 10th gear. I mentioned 55 MPH because I found on secondary roads the transmission would shift into 10th somewhere between 50-55 MPH depending on the load. So for folks that want to get the highest fuel economy, they would probably want to get into 10th gear once they fully get on the highway.
You could run the trans in L9 all the time, as mentioned, there it very little ratio change between 9th and 10th. I would say it is closer to 200 RPM as I recall when I was closely monitoring the gear and RPM differences. It is pretty hard to tell when the transmission shifts from 10th to 9th unless you have a scan tool that displays the transmission gear which I had been logging for a while. I have not looked really closed about the overrun clutch as mentioned, probably would hold back a bit more on decel if the case.
You would take a slight hit in MPG, I think I can get close to 500 miles per tank if I pushed it, but for me a vehicle is out of fuel at 1/4 tank, this is the world I live in.
7th gear is 1.00
8th gear is 0.85
9th gear is 0.69
10th gear is 0.64
As mentioned the 4L80/90 4th gear is 0.75, so this is actually between 8th and 9th on the 10 speed.
9th will clearly help offload a lot of the low RPM, high Load that the 10 speed puts on these V8 engines. But the 6.2l TCM holds the gears much longer and loads the engine far more than the 5.3l TCM. Running in L9 might actually save the 6.2l from itself in the long run!
