So i bought a 1983 280zx the other day, and when i started going down the road i realized that my digital dash speedometer lit up, but it DIDN'T SHOW THE MPH How can i resolve this issue, what can i look at to find out what i have to do?

 

hard to say, but if memory serves me correctly, you still have a speedo cable going to the cluster, so I would start there. Check that the speedo cable is in fact turning the chopper wheel in the cluster.
Other problems include electronic issues that can be repaired, and after all this timr since the date of manufacture, I would look at replacing all of the electrolytic capacitors on the circuit boards in the entire cluster.