I was puzzled by this myself. The speedometer has little markings ("I", "II", "III") at the shift points of the various gears and a "red" zone (It's yellow on my car) right after the "I" marking, which corresponds to the red zone for first gear. I'm not sure why they didn't put it on the tachometer. BTW, the engine cannot over-rev. Something in the injection pump prevents that.
As for good cruising RPM, I don't know.
1982 300D - <A HREF="http://members.xoom.com/nicodore/