Whats up with my speedo?

:blob:I recently replaced my 904 trans in my 75 Duster, I got it all in, runs great. But my speedo jumps like crazy. It did this even before I put the new trans in, I just thought it was because that trans was going. But I guess it had nothing to do with that. The gears for the speedo in the trans are fine so I'm not sure whats going on. Like I said the speedo bounces litterally it will not sit still, say im going 30, the speedo will bounce from 20 to 45 non stop. I have no idea how fast I am going till after 50 mph then the bouncing isnt as bad, it just stays within 5mph either way of my speed. Whats going on?