I keep records on all my cars including; odometer reading, trip odometer reading (reset at every fillup) and gallons taken to fill. I can support 26 and 27 mpg from my 300E, however A. It's a manual transmission, and B. it's 90% or greater road miles, most with the cruise control set in fifth gear. However, the times I have driven heavily in the city, I've never dropped below 20, and I'll be surprised if a tour through the logbook shows any tank below 22.
People laugh at my record keeping, but it gets my attention when my fuel mileage falls off. I usually can home in on something before it gets worse.
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in