Yes, lugging a diesel is bad. But let's make sure we're lugging. Most people shift down too early, either from lack of knowledge or failure to plan ahead. Wears the clutch, transmission, and fuel budget unneccesarily.

Coasting downhill is not lugging: you have your foot off the throttle and you're letting the engine's compression hold the vehicle back.

Cruising on the flat may or may not be lugging. If so then you'll feel the engine struggle, see smoke, and see the engine temperature go up. Here's the trick: keeping the throttle steady and no car complaints means you're fine. Adding more throttle and the car complains means you're lugging. A real light throttle addition and no complaints means you're fine, although it may take until next week to get up to speed. daBenz will easily cruise at 32-33mph in 4th gear, but gets shifted down a gear when it's time to accelerate. My last road truck would let me cruise three gears higher than the gear I needed to accelerate. Try this on the way down: coast until the car chugs (you'll be amazed how slow it can go), then shift down. You may even be able to skip a gear if you need jackrabbit acceleration in a jam. Practice makes perfect.

Thinking and planning ahead makes a happy diesel.

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daBenz - 1970 220D