Comon problem. BTW, the speedo is not mechanical, ie. no cable.

Do a Google for '850 odo repair' and there are a few good ones.

To remove the instrument panel, just lift up the left side to get the cluster out:
http://volvospeed.com/volvo_repairs_how_tos/volvo_trim_parts/dash_pad_removal.html

Leave the car in the sun to get the dash warmed up so it bends easily. The vents really do pop out without breaking :>)

Use the "A" box to get the current mileage. A-7. Push the button 5 times, wait a second , the 1 time, wait again, then once more - that gets you the right three odo digits. 555-1-1-2 will get you the left 3 digits. Just count the blinking lights. A zero will be 10 flashes.

Use a drill and a pencil with eraser to turn the odo to the proper mileage.
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My name is Klaus and I am a V ♂ lv ♂ holic