busted speedometer

so my alternator gave out on me yesterday while i was driving and drained the battery like there was no tomorrow
all the gauges went haywire and eventually died out on me
car died right as we pulled off the road
figured the alternator was bad, hitched a ride to autozone, grabbed a new one and got it all fixed up, everything was fine
started up the car later that night and the speedometer jumped

speedometer is now stuck at topped out(whether car is on or off)
any ideas?
any help much appreciated.