speedometer is eratic

i have a 91 gt, i just recently got it together and started driving it, i noticed on the first time down the road, the speedometer was running faster than acual speed and bouncing around, the road i live on the speed limit is 45 and when i run what i feel to be 45 the needle will run up and jump between 60 to 70 mph, i pulled out on to the highway, and the speedo went up to 110 mph as i got passed by a semi in a 55, then the needle came back down and bounced around 80-90 mph, i went back home, left in it again few hours later, went down the road, the speedo was reading 45 but with a bounce once in a while, went out on the highway, was reading 60 mph, like it was working now, on the next drive it started doing what it was doing on the first drive, and has been doing it every since. any ideas on the fix would be helpful,  

you might check your speedo cable and speedo gear in the tranny also check to see if cable is pluged all the way in the back of your speedo  

would it make and differance if it is an automatic car that has had a 5 speed put it? because i thought the auto cars were geared diffrent than the 5 speed cars?  

i've had 89 LX AND I GOT A 91 LX NOW AND BOTH THE SPEEDOS DID'T WORK RIGHT.I JUST USE MY GARMIN FOR A SPEEDO.  

the cable not ran right made sure its not making any tight turns