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)
They are shaped a little different at the bottom around where the steering cloumn goes through, like mentioned above. Plus,the printed circut board on the back is different in someway. I tired a 90 cluster in an 87 4 cyl swap and it wouldn't work. So instead of getting a new cluster, I swapped out the circut board on back it it worked. Also the multifunction swtiches are different from 87-89 and 90-93's. Found out the hard way on those also...