I'm still working on my '87, but my first couple of rough calculations has me seeing 28 mpg city. I hope to get into the 30 mpg territory for city use and some crazy numbers for highway. 5th gear helps a lot.

Swapping out a flex disk/shifter now

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08 R320 CDI current

Past
95 E420
87 300D Turbo 5spd
90 300TE
83 300SD
85 300TD
92 400E
85 190D

My 190D auto with 2.2 na has averaged right at 36 mpg over 5,000 miles. Running over 65 mph loses about 10% in fuel mileage. Best tank was 38.6 and worst was just under 34. I could see a 5-speed with more favorable overdrive gearing (auto is 3500 rpm's at 70 mph) could easily push you into the low 40's.

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You can't always get what you want, but if you try sometime you just might find you get what you need.

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