Up until 1989 Ford [was required by law] to use an 85MPH speedometer in their cars. The law expired in 1990, so somewhere between 1988 / 1989 Ford began making the switch to the 140MPH speedometer. My car was manufactured 1/89 and came with the 140.
Mechanical and electrical differences, I don't think there are any (other than getting the speedo re-calibrated)