Hi i'm looking for some help troubleshooting a overactive speedometer on my 89 Chevy Van. It reads about 35mph to fast, so when i'm driving about 35mph the needle floats at about 70mph. Also when parked with the motor shut off, the needle sits at about the 35mph mark instead of 0 where it should be.

It was working how it should be about a week ago and this happened while I was out on a drive, it actually seemed to occur right after I was doing some reverse maneuvering in the van trying to back out of a place, if that helps figure out anything. Odometer still works fine. If anyone has had any experience with this type of thing I would sure like to hear from you and what i'm looking at to fix it, i'm hoping to be able to fix it without having to take it to the shop. Thanks!