Unusual speedometer issue

[BrownBetty]

I bought a 1980 300TD wagon about six months ago. The previous owner warned me that the speedometer was a little off, I just didn't know how bad it was. I searched to see if anybody else has had this issue, but I can't find anything. The speedometer is inaccurate at all speeds. I've done a few tests with roadside speed displays and I usually just drive about as fast as the cars around me to estimate my speed.

Speedo / Actual speed:
10 mph / 10ish mph
20 / 25
25 / 35
30 / 42ish
35 / 50
40 / 60ish
45+ / fast

The speedometer caps at 45 mph, but I'm going way over 60. I've been ok so far because I drive mostly in the city where there are other cars to compare speed with, but I know I'll get unlucky one day and get a fine.

The odometer works, I've tested it against gps. The previous owners worked on the car some, so it's a possibility that the instrument panel has been replaced. I'm not sure if the problem is with the speedometer itself, or the cable. Maybe there is an issue with springs or worn out parts in the speedo. Or something else?

What do you think? Let me know if you need any more information.