It does sound like something is slipping.

If you were to put the measured rpm (and tyre size etc) into an online calculator would you then get the correct road speed for every gear?

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1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

Did you measure the torque converter to see if it is in spec?
In the MB rebuild book, there is a dimension you need to measure to see if the TC is damaged and expanded. A expanded TC will cause higher RPM's then a good one because the internal distance between turbine vanes and stator are too big.
This will however be noticeable in all gears, but none the less, overall RPM's will be higher. Funny thing is, the car might actually feel sportier with a bad TC because you now theoretically coupled the engine into a higher power range. Just like a racing torque converter.

Can you force the car into 1st gear? I don't know about the w140's, but the w126's have a indent in the 2nd gear shifter position to activate 1st. 1st gear is electrically activated, unless you really stomp on it from a dead stop either with or without hitting the kick down button.

Lets just make sure you are starting off in 2nd and not first..
Because from that video, it looks like its in 3rd, and around 0:53, it downshifts to 2nd.
If that's the case, totally disconnect your boden cable, and manually shift to ensure you have all gears. Car will almost instantly shift into 4th upon takeoff. Scratch that ideal, the w140's are vac activated boden...

Also another thing you can do first is, disconnect the single power cable going to the first gear/kickdown solenoid on the back of the trans. I think your w140 has the downshift button behind the gas pedal. This might be faulty, and causing the trans to always be 1 gear behind. Its could be either that, or a bad internal valve inside the solenoid. This can easily be removed from the trans and bench tested with 12v to see if the valve properly opens and closes. Now that im thinking about this, I suspect that's whats going on.

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Just my MB's:
1990 420sel - Winter DD - 410,000km
1990 560sel - absolutely showroom mint - 360,000km!