Having owned and operated a C230 for five years, here's my thinking...
... driving the same machine 15,000 miles per year over the past five years (without fuel gage issues), here's what I can tell you....
For typical surburban duty, tank after tank, my C230 returns approximately 27 miles per gallon.
(Sorry, I too lazy to do metric).
Throw in a 500 or 1000 mile highway trip, and I get an approximate average of 37 miles per gallon for the trip at approximately 70 miles per hour average (higher at slower speeds).
The freaky party is that upon returning to surburban driving (after the highway trip), I get way over my average surburban 27 miles per gallon (closer to the highway mgp figures). It usually lasts for about a tank or two.
The efficiency of this machine never fails to amaze me.
1988 300 SL (5 Speed)
1994 E320 Wagon