My 74 240D has had some poor fuel economy lately. The odometer doesnt work, so I dont really know how poor it has been, but it seems to be less than before. There are not any other visible or noticible problems other than that. Any ideas on what this can be? Thanks

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74 240D
84 300D Euro

Inspect your air filter. If dirty enough will affect milage. Will be hard for you without a working odometer as you are just going by suspicion. why not grab a used speedometer head at the wreckers or fix the old one. Lots of threads on this site about broken head units.