1993 Ford Explorer My car

If your transmission uses a speed sensor that's driven by a gear, the gear may have the wrong number of teeth. Tire size could have been changed. You didn't include any idea of how far off the speedometer is. Is it one or two miles per hour? 20 mph? Also, if it reads five miles fast at 20 mph, does it read ten miles fast at 40 mph or does the amount of error always stay the same? If it stays the same at any speed, the pointer may be loose and has rotated a little.

If you have an electronic speed sensor, the tire size and axle ratio have to be programmed in for the speedometer to read correctly. You'll need to find a mechanic with a scanner to do that.