speedometer reading too high and too low

1997 Ford Explorer - speedometer inaccurate - reading too high and to low. Needle does not bounce while driving but cannot get consistant readings.

What might the problem be?

Update- this is my daughters car and she is now telling me it only reads too slow.
It will show 55mph when she is actually go 60-62 mph

Thanks!

this could be a wheel speed sensor problem its located on the top of the rear end obove the pinion yoke this troble also happen in the f pickups

In the 1997 Explorer the speedometer voltage is generated by a vehicle speed sensor attached to the transmission, not sourced through the ABS system. How has she determined the speedometer reading is off? If she is are basing this on a GPS speed signal, she needs to be at a static speed on level ground for several seconds before making comparing the measurements.

Check to make sure the tires are the correct size and are inflated properly according to the door sticker (not the tire sidewall). If the tire size is incorrect or the tires are not properly inflated, the speedometer reading will be off. Even if everything is correct, a reading of 10% error may not be all that uncommon, but usually the error would be in the other direction (reading higher than actual speed).

-Rod