Could be a couple of things : #1 wrong size tires on car #2 wrong speedo in car #3 bad speedo or sender One way to check if your speedo is reading correctly is via highway mileage markers(assuming your state has them). Keep at a constant speed. For the sake of keeping this simple lets say 60mph. Which would be one mile per minute. Take note of time(to be super critical use a stopwatch)it takes you to go from one mile marker to next. Should be exactly one minute. Wish I could offer better/more help but do hope this helps you somewhat.
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' 88 740GLE auto wagon nearly 167,000 and ' 87 240DL auto sedan nearly 125,000