Straight from the Robinson Racing site:
http://www.robinsonracing.com/tips.html
Question: I am currently running 48 pitch gears and I know the tooth sizes that perform well in 48 pitch, but I want to change to 64 pitch. How do I convert the the number of teeth on gears from 48 pitch to 64 pitch?
Answer: Good question. The short answer is: take the number of teeth on the 48 pitch gear and divide that number by 1.5 and then muliply that answer by 2. Here's and example:
(81 tooth 48 pitch) 81 ÷ 1.5 = 54 x 2 = 108.
The answer of 108 will be the tooth size to use in 64 pitch. In that example we get a whole number after converting, 108. Not all of the numbers will work out this cleanly, lets take an 86 tooth gear for example:
86 ÷ 1.5 = 57.333 x 2 = 114.666.
We need to round this number off to the nearest whole number, 115 (a common 64 pitch spur gear size).
Just reverse the equation to convert 64 pitch to 48 pitch. 108 ÷ 2 x 1.5 = 81.
Therefore:
96t ÷ 2 * 1.5 = 72t 48 pitch