I recently purchased a 1958 F100 with a 1970 351C and C6 trans. I installed a speedometer cable in place of the broken one. The speedometer works well but is reading about 20 mph too fast (80) at approx 60 mph. The current gear is 16 tooth blue/green plastic and is about 2 inches long. I ordered a replacement 20 gear black plastic gear and it is only approx. 1 inch long. It does not work, the gear seems not to make contact. Any ideas? Thanks