i have a 1980 ls my tachometer has the voltage buit into it and shows voltage when the ignition is "on" but when running the rpm's are always at 0 possible cause??

ALSO!!! my speedometer works...but is very off...for ex. 15 MPH is 55 on my speedo it goes all the way to zero when i stop but when i accelarate it makes my car look like i can do 0-60 in 3 seconds...( i wish)