Changing the rear gears should not make the speedo jump around but will change the reading. You will be going faster then it shows. Remove the cable from the speedo head & pull out the core. Feed the core back in with some grease. Put a small amount of oil on the end of the speedo head where the cable goes. Make sure there is plenty of grease in the trans. This should make it work smoother. You may need to get a speedo gear with 1or 2 less teeth.
