This is a common problem. Cracked odometer gears are the cause. Your Speedometer will continue to work fine. Let it go as long as you like, unless you use it to keep track of your gas mileage, oil change intervals, or something like that.
See the thread HERE
for pictures, a how-to and where to find the replacement gears.
Check out the how to at:
How-To Repair Odometer Gears
This is the best How-to I've seen for this repair - complete with lots of pictures.
I wouldn't recommend removing the plastic cover before removing the instrument cluster as you might damage something. It's a little tricky to unclip the wire connnectors but it can be done with a little care.
This is the How-To that I used to repair mine. If you mark the speedo needle and put it back properly there's no need to recalibrate. I did mine in February and it's working fine.
I got my gears from http://www.supercoupeperformance.com
for the same price as odometergears. Their gears are blue and look - at least to me - to be a better design than those at odometergears.com
And whatever you do don't reset the trip meter while you're driving the car - especially if your going fast. This will strip the gears and fast - and you'll be doing this repair all over again.
I'm not pushing SCP or anything just offering another option.