The usual cause is a bad speedo cable. They get dry and start to "wind up" then over-run when they spring far enough. They can be re-lubed, but by the time they start making the needle jump around, it's better to replace them.

Also, the bearing in the speedo head can get sticky, too. This is more expensive.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!