I had someone with a working speedometer drive head of me and took 5 mile per hour checks of what my speedometer read vs the correct value. On MS excel I entered this data and formed two grafts one of my reading and one of actual speed. From this I was able to tell my speedometer was 15% off. By increasing or decreasing 1 tooth in the trany speed gear, you adjust the reading by 5%. So I needed 3 teeth. It works great now. Good luck.