I obsessively calculate the fuel mileage using the odometer and gallons to refill the tank method, been doing this since I was 16 (my friends used to think I was weir, but now that we're all over 40, they understand). I put about three hundred thousand miles on a couple of 1992 300D's (2.5 5 cyl OM602) and almost always got the same mileage: 26-28 around town and 33-34on the highjway, and anywhere in between if it was mixed driving. I was once able to get 36 mpg on a tank when for kicks on a cross country road trip I drove at 55 mph the whole tank with the cruise, no a/c, windows up, and on summer diesel. Normal 70 mph cruise yielded highway 33-34 mpg like clockwork.

I drove an '87 300D for about 30,000 miles and calculated the mileage every tank. Mpg figures were across the board about 3 mpg lower for the 3.0 6 cyl OM603 in the '87 300D. That is, about 24 or so around town, 30-31 on the highway, but could squeeze 34 or so when "hypermiling" on the highway.

These figures are with very good running cars with good fuel, tires, clean injectors, etc.. I had very consistent mpg figures, but as they say "your mileage may vary."

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1972 280SE 4.5
1984 300SD, 250K
1991 300SE, 89K
1992 300D (330K miles when KIA'd by mother-in-law 8/10/09 - still sitting in my barn)
1994 E320, 109K
1995 E420, 60K
Proud Member of the Mercedes Benz Club of America - Idaho Chapter