*carefully* remove the VS from the diff housing, and verify the plastic speedo ring gear is secure on the diff. If it's loose, it'll spin, and your speedo will either not work, or read very low. if it feels secure, *carefully* remove the pinion gear from the speed sensor, turn the car to ON (but not running) and spin the speed sensor in a cordless drill. if it reads low or doesn't read at all, you have a bad VSS.