help speedo

right my speedo kinda works but when you stop stays where it was but when you mess with back it goes back to 0 but dont think its the right speed it really  

3 parts to the speedo

Drive gear in gear box ?10.00

Speedo cable ?35.00

Speedo head   ?120.00

Start with the cheapest item ? My speedo did that , it was the head  :evil:

it might be at the back of the clocks that its not poped home ive had same probs when i have had dash out

thanks but dont think it that as i got three sets one speedo works ok and other two dont one that works is plastic where cable goes and others are metal but on one with speedo working nothing else works tried to take apart but carnt as bord on back breaks when i try