The car in question is my 1989 900S. Typical VDO mechanical speedometer from this era.
I had the instrument cluster out of the car (again) yesterday to deal with a few things, including the odometer (again). Prior to this latest R&R, the speedo read ~10mph high at 60mph (so a reading of 70 meant actual speed was 60). Now it's off by a greater margin, more like 15-20mph high (so 75-80 at 60 mph actual). I marked the needle position when I removed it and returned it to the same place, so I don't think that's the issue. The plastic clip for the speedo cable is cracked (naturally), but I glued and sleeved it so that it holds the cable on without a problem. Other than that, everything is as it should be.
What I'm thinking is that the speedo needle return spring may have weakened over time and is allowing the reading to climb faster than it should (why it's worse all of a sudden, I don't know). I may need to pull it yet again and reposition the needle to put more spring tension on it, thus lowering the indicated speed, but before I do I figured I'd throw it up here and see if there's something I'm missing. Any other things I should be considering or inspecting to get the speedo closer to reality?
