Since getting a 1991 300D 2.5 turbo earlier this year, I have averaged about 35 mpg. On a road trip, I got 38; in town, in the lower 30's. It has 100k miles --just a baby, yes?
I don't know where you got the specs --I have seen 26-40 listed (in the MB Buyer's guide).
This car is sooo much more civilized than my (beloved) '79 300SD, which, with 360k miles, still gets 27 on the highway (albeit noisily).