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Yeah, I have the same problem with my '84, it gets 20mpg routinely. Then on one tank it got 24mpg! Then back to 20. (No, it wasn't bad math, that tank went over 400 miles where normally it gets to ~350.) Fuel quality can make a big difference, try a different diesel station. A workmate has an '85 300D and he gets 30-31 on the freeway (light foot).
There are mechanical things that can affect fuel conusmption but normally they also make the car run poorly. Try checking tire pressures, and make sure none of the brakes are dragging. Also check engine oil level, diff oil level, and trans fluid level. Make sure your air filter & fuel filters are clean too.
When was your last valve adjust? They're supposed to be every 15kmi, IIRC. Is your EGR valve functioning properly, or correctly disconnected? I've heard of people getting several more MPG by disabling the EGR.
If you figure it out, let us know. I'd like to hear what the cause is. Maybe start a new topic, with your specific year+model in the title, like "1984 300D - Runs Great, Crappy MPG - Why?"
:p
Good luck,
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