Need help. I have a 70 mustang fastback, 4 speed car. The speedometer was reading about 10 mph slow at 60 mph. This was the case since I bought the car 2 years ago. I pulled the speedometer gear out and reinstalled it. The speedometer then registered correctly except it jumps around. I installed a new 20 tooth gear, exact match to the old one. When I was testing it out today, I was in 3rd gear, speedometer showded 20 mph, when I took my foot off the gas pedal the speedo jumped to 35 mph. This seemed like the correct speed. When I lightly touched the gas pedal again, the speedo dropped to 20 mph. I did this several times, same thing. It seems like the speedometer changes with the movement of the gas pedal. I have looked for any interference with the cable, but can't find any. Any ideas?