speedo blues (or joys? :) )

Ok, my speedo in my 99 nubira went nuts the other day and while I was only doing no more than 5 mph (sedate pullout from the gas pump) it jumped up to WAY past 140 ! and now it's perpetually stuck doing like .... I would guess 155-160 mph lol It was a funny thing I actually had fun showing it off until yesterday when I got a speeding ticket for doin 80 in a 65 zone... not funny when the cop asks you "you know how fast you were going" and you have to reply... ummm my speedo sayd 160, see? :)" so now I have an equipment failure ticket as well >.<
what could have done this and how to fix it?

have you checked the battery? When you turn off the car, does it get back to 0?

nope. it sits there just buried. i even disconnected the battery, turned the ign a few times and reconnected the battery - no joy