1993 f150 4.9l spedometer?

i got 4.9l 93' 4x4 f150. my spedometer is either 5 miles off some days or 2 or 3 mph off other days. i know this when i go past the speed limit reader signs cops put up in my area. i know their is a speed sensor located in the rear end because it has ABS. could this be why it is off by 2-5 mph?