What would cause the speedometer to not accurately display MPH? on 1990 Mercedes-Benz 420SEL

Example: If I am going 25 MPH (as shown on speedometer) my actual speed is approximately 45 MPH. What is causing this to occur?

Asked by for the 1990 Mercedes-Benz 420SEL
It sounds like the speedometer on your 1990 Mercedes 420SEL is reading in kilometers per hour instead of miles per hour. If there a switch located some where on the dash to convert the reading from metric back to english units?
1 more answer
Thank you Jim,
I do not see any switch that can be set to kilometers or miles (unfortunately). I may scan the trans to see if the sensor that is in there is for K instead of M. Thank you for the advise.