The assumption on the fuel gauge is likely the case, there has been a fairly lengthy thread on that very subject recently. Just use the search feature of the forums. The tach problem is often the little round black cylinder on the drivers fender well under the hood. Again use the search feature, as there have been many postings on that subject!! Why not keep the wrecked CD as a parts car yourself, seems like these items you are wondering about worked on the old one???

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1961 190Db retired
1968 220D/8 325,000
1983 300D 164,150