Not sure about the wagons but my '84 gets about 25mpg. Its got about two degrees of chain stretch so the cam and pump timing are pretty close. I just did the valves, fuel filters are good but the air filter could stand a change. That mpg figure was averaged this January so I haven't had a chance to check it on winter fuel. My '85 got about 21 and I also think that might have had something to do with the timing chain. It was at least seven degrees stretched and it would stumble for about a minute on cold starts, whereas my '84 fires right up.

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Seth

1984 300D 225K
1985 300D Donor body
1985 300D Turbo 165K. Totaled. Donor Engine. It runs!!!
1980 300SD 311K My New Baby.
1979 BMW 633csi 62K+++? Dead odo