This is a pretty common fear among new 123 owners... Its normal that it sounds like it's revving high on the highway. Takes some time, but you will get used to it. As long as you feel it shift through all the gears, and you know that it is in 4th when you are on the highway, and it is clearly not slipping, you should be just fine.

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1981 300CD 260k w 60k on new motor
1983 240D 4spd Manual