speedo right a low speeds of at high?

my speedo on my 85 500sel euro is right at anything under 35 or 40. After that espically on the interstate it will say i am doing 80 and have got clocked at 90. what could cause this a bad cluster or something like that.
thanks

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Dan Younes
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1985 euro 500sel 223k
1986 420sel 86k soon 2 be
1981 300sd 285k sold
1979 240d 298k sold
1983 300sd 272k sold
1989 chrysler 2.2l Turbo
Leabaron 140k
1989 Dodge Carvan Se 295k
1994 S500
(hopefully someday)