I have a 1973 Mustang with a C6 transmission, 245/60R15 rear tires and 3.25 differential. How do I calculate the correct number of teeth for the driven gear? I have seen one source that says the C6 transmission has 6 teeth on the drive gear but cannot confirm this. How many teeth do I need on the driven gear and what is the formula. One equation reference calculates 21 but other equation references calculate 15. The original gear has 18 teeth. It seems to me that I need more teeth.