Trouble Shooting the Speedometer on my 71 C3 Corvette

I installed a new speedometer in my car about a year ago along with a new cable, plastic gear and steel gear in the transmission. The speedometer needle bounces from 0 to 20 mph until the car goes above 20, then it stays pretty smooth until about 45 mph. The needle bounces a little until around 60 mph then it bounces between 60 and 70 mph. This video is me testing the speedometer to see if it is bad. Disclaimer: The work we do around our mini farm and our vehicles is for entertainment purposes only. This is not how-to advice.