A very doable fix if not totally pooched.You can start with this thread:
I used the method by JohnM to fix both my speedometer and tach.
It took a few times to get it close enough.
Just remember the conversion of speed vs tach in 5th gear is 22.14mph per 1000rpm. Warm idle will be around 800rpm.
That is if your speedometer is accurate.
I checked that with a GPS.