My initial question to myself was "How did you test your spedometer? I would take it to an expressway where there are milage markers and then take readings on your odometer as you pass a series of markers...say a reading at each mile and then do an average at 5 miles. If your odometer agrees with the markers then there should be nothing wrong with your speedometer. To throw your speedometer off something significant must have occured over the history of the car. Like a new rear gear box with a different gear ratio.
If your speedometer is electronic (not mechanical cable) you might check the electrical cable connector where it plughs into your gear box and check for corrosion or poor contact.
1993 190E 2.3
2000 Toyota 4x4 Tundra