David,
Your mileage does seem a bit low, but not out of the norm. Every car and driver combo will get different mileages per tank. My wife can usually avg 22-25 in her 300D, but I can avg 25-30 MPG when driving the same roads.

Get a tune-up (filters, air cleaner, injectors cleaned and adjusted, etc.) This should help.

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William
76 240D (W115) - 550K miles
78 300D (W123) - 200K+ miles