I have a record of every gallon of diesel used and the mpg for every fill up for the past fourteen years and 150,000 miles. I have it for thirty-two years on my 240D. IMO the only way to get really accurate as to what your mpg is, is to do it over several tanks of fuel and several thousand miles. I don't plan to add up all the data for mine, but suffice it to say that the average for my 210 is in the upper 20's and for my 240D it's in the mid-20's. My 210 has shown a few as high as 34 mpg, but the next tank would usually bring the average down to more normal numbers either because of foaming fuel, filling up on an incline or whatever else may have affected it.

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Len
'59 220S Cabriolet-SOLD and living happily in Malta
'83 240D 351,500 miles original owner-SOLD
'88 560SL 41,000 miles - totaled and parted out
https://sites.google.com/site/mercedesstuff/home
'99 E300 turbo 227,500 miles
'03 SLK320 40,000 miles - gave to my daughter
'14 Smart electric coupe 28,500 miles
'14 Smart electric cabriolet 28,500 miles
'15 Smart electric coupe 28,000 miles