Ok so it is only for diagnostics then, good to know. I thought the car might have been in some sort of "limp home" mode or something. I have no warnings to indicate this. So since it seems to be running ok I will assume it is. I guess I will start tracing the tach problem after I get the clutch replaced.

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Currently
1991 Mazda Miata 174000
1991 Subaru 189,000 Winter beater

got rid of:
1984 VW Vanagon 195,000
1976 350SE Euro 185,000 KM
1973 280SE 4.5 (boy do I regret that)
1980 280SE European W126 4 speed manual (miss it)
1989 BMW 735iL 119000 (good riddance)