OK missed that.
We will need to know how far off the Odo is and try to calculate expected MPH offsets.

Mile markers should be more accurate than GPS. I'd expect GPS to give straight line distances not taking into account hills. ( At least the consumer versions )

Wrong speedo .May have been changed over time, with one that just looks the same . Get the number from another member .But make sure you car is the same specs as his w123 you then can purchase the right one And then you will know if it was replaced .. The number will be on the speedo dial at the bottom .. Try a spot of oil in the speedo cable first it might be sticking .