when driving at 115/1118km/h and engine turns at 1700/1800rpm, the car often stays in 8th gear.
i CAN manually put in in 9th (engine drops to 1500rpm) and when i do nothing further, the car wil go back in automatic and stays in 9th gear

But when i manually put it in 9th and then manually switch back to 8th, the car advises me to shift to 9th.
and when i don't do anything, the car wil go back in automatic but stays in 8th.

similar behaviour when i'm in 7th (arnd 80/90km/h) , it won't go to 8th (but i can do manually)

I'm guessing this is not normal, cos if it was, the car would not let me shift into a gear which isn't adapted to the situation ?

2 clips:
https://www.dropbox.com/s/wqy1jxuphz...champ.mp4?dl=0
https://www.dropbox.com/s/9xdel1x713...champ.mp4?dl=0

Normal as the car will have worked out you are using less fuel with a lighter throttle and more revs to maintain speed. The lowest mine will go into 9th and stay there is 121 kph as lng as it is very flat. Can gernerally do it at 123 (which is a true 120) but slope/wind dependent and a very ligth toucvh on the throtle. In UK at 70 mph indicated the car will always be in 8th.

thanks , that seems the same behaviour as my car,
but doesn't explain why the car would advise me to go into 9th when in manual (when in automatic it stays in 8th)...?

My 2021 E350 (Gasoline) switches to 9th gear at about 113kph. Might be difference between gasoline and diesel.

I think the rason why you can manually put it there is because it is technically posssible, but when in "D" the car brains will switch to 8 as it is more economical (engine would otherwise be labouring). You could put my old Mondeo diesel in 6th at 35 mph and wait 30 secs to get to 50, or you could put it in 3rd and be there in 5 secs

You might be right, but why, when I switch gear from 9th to 8th (the gear the car was in when in D) , it advises me to switch back to 9 ?
i just find that 8th gear is "smoother/quieter" than 7th when driving 90km/h and 9th gear "smoother/quieter" when driving 110km/h

For what its worth, just yesterday I was cruising on a large city road at 50 mph and I noticed I was in 9th. I was in D and in Comfort fiddling with the consumption gauge. It is also important to note that I have been trying to figure out my worse than expected mpgs so for the past few days I have been driving ultra conservatively and trying to hyper mill a bit just to see if I can get the thing over 22mpg. I am sure this messes with the mapping a bit.
BTW, if you want to get a reasonably accurate mpg estimate for shorter drives you can reset the trip computer for "this drive" and it will reset the consumption counter so it starts at what you are currently getting instead of starting at 0mpg and sloooooowly creeping up.

I had a 2021 E350 which didn't go into 8-9 until 75mph. It also jerked between 2-3 gear. I took it in to try to fix - they loaded new software, but it didn't fix anything. I found a new 2021 E350 which had the options I wanted, and colors I wanted and traded the old one in and the problem went away in the new car. It shifts to 8-9 at around 60mph (on flats or downhill). You could try a transmission reset (which I found helped a bit for the problems):

If you have KEYLESS GO do not remove the Start button.
1. Press Start button twice. This sets it to key position 2.
2. Floor gas pedal for 10 seconds.
3. Press Start/Stop once. This turns it OFF.
4. Release gas pedal and wait two minutes.
5. Then start the engine and begin driving.

Other option (not KEYLESS):
1. Turn the ignition key to the ON (not start) position.
2. Press the gas pedal to the floor and hold for 10 seconds.
3. Turn the key to the "off" position (don't remove the key), then release the gas pedal.
4. Wait at least two minutes for ECU to reset. When the 2 minutes are up take your foot off the accelerator and start the engine. Drive away and you should notice a more responsive transmission for a while.