Nissan Nomad help.

Hey all, not sure if I can get any help for this here but lets see. The speedo for my nissan nomad (1990) is really inaccurate and slow to respond to speed changes. Is this something I can fix myself (I'm fairly handy with tools and electronics and don't want to go to a mechanic as I'm on quite a budget) and can anyone here guide me as to what might be the issue?
Thanks in advance for any help!
Nat.