Speedometer gone nuts.
 

1994 GMC 3500 dully 6.5 with 220,000 miles The speedometer is reading 0-85 mph without truck moving.

clean all the grounds first

My truck also does it but only goes up to 10 mph. Common chevy thing.

Mine did that for awhile, but now it is fine, the only thing i can think of it mud or dirt messin it up