My 85 190e had the same problem. I finally fixed it by buying a used speedometer. I though it was the cable too, even bought a new one, but the speedometer was bad.

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1985 190e 2.3 5spd
98 E320 rims + TV & VCR
1991 Mazda Miata
1991 Yamaha FZR1000
1994 MB S420 (sold)
1994 BMW 318ica (sold)