300cd, the short answer: probably not. As I re-read your first post, I now wonder if you're shifting at the dots or dashes on the speedo. They're there to show you the engine red-line for that gear, not to show you where to shift. MB was smart that way; you don't need a tach. The real answer comes from the torque and horsepower curves for the engine. The old rule is to shift up at the rpm where the engine rpm is at 60% of rated torque, and not run the engine faster than 80% of the rpm at rated horsepower. Sounds confusing now but will make sense when you see the curves. A good test for lugging on the flat is this: if the engine stumbles when the foot throttle is lightly applied then you're lugging. Also watch the temp gauge when climbing hills. If it begins to creep up then you're lugging: shift down, drive at 80%, and enjoy the view after the temp comes back down (it will, and quickly if you catch it fast enough). daBenz will pull any hill in 3rd at 45mph and not heat up, which isn't bad for a 60HP engine.

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daBenz - 1970 220D