97STANG

Anybody know the part number or which gear to get to correct my speedometer. I have a 4.10 and I am just sick of the speedmeter reading so far off. Not to mention racking up "fake" miles. I believe the sn-95s actually have a mechanical gear in the tranny, its an auto. And the 99+ have some sort of a speed sensor??. I called the local ford dealer and the guy said he couldnt find one? Any help would be great!