tachometer trouble
 

so im driving my dads '91 dodge dakota, i was taking a friend home yesturday after work and she was tlakin about being so tired and was gonna sleep on the way home, well when she leaned over and closed her eyes i just wanted to mess with her so i drove over the little ridges on teh side of the highway lol, well when i did this my tachometer shot up to 4500 rpms and never went back down. does anyone know what might have happened to cause it to stick? it doesnt go down when the truck is off and doesnt move at all if i rev the engine. ive looked all over for some idea of what might be wrong or how to fix it but havent found anything. thought id see if someone could help me out.

You could have jarred a ground wire loose. My tachometer has a mind of its own, it jumps around or drops down to 0 and stays there for a while, but it works about 85% of the time and I think my issue is a bad ground. The thing I don't understand about your problem is the couple times I've taken my dash apart, I've removed the tachometer and the needle is very loose and sensative, so I would think if it were a bad electrical connection, gravity would pull the needle down, but the ground connections behind the instrument cluster is where I would start looking and then go from there.

They also use a tachometer module (mounted on the instrument cluster) which could be bad.
But, sounds like something electrical, since it happened just than.

Mine kinda jumps around and goes dead sometimes too...

when my truck is idling, my tach jumps from 200RPM to 750RPM or so on its own....