What's the gauge cluster out of? IIRC, the 81 300CD didn't come with a tach. Are you still using the 3.46 rear end that the car came with, or did you swap to something different? As mentioned above, the speedometer is matched to the rear end ratio.

As for the tach, I can't help you. Mine does what it wants, including reading 6000 rpm at idle, 0 rpm when cruising, randomly reading 500 to 1000 rpm high, or sometimes even seeming to read accurately. It's getting slightly annoying, but there are other things higher on the priority list: shift bushings, fan clutch, slave cylinder, driver seat re-cover...

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.
99 Mazda Miata 183,xxx miles.