Check your VSS (Vehicle Speed Sensor). I have found most speedo problems stem from a bad VSS. Other symptoms are rough shifting and O/D flash. The sensor is located on top on the rear diff. check out this post.
http://www.ford-trucks.com/forums/51...ed-sensor.html

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(SOLD) 2003 Ranger 4.0L V6 FX4
2001 F150 Supercrew Triton V8 (DD)
2008 Fusion 3.0L SEL V6 (my baby)
1992 Taurus SHO MTX (my toy-almost Supercharged)
2004 Nissan Murano SL AWD (Wife's car)