Originally Posted by
YMCA
53x13 = 49x12 (4 teeth)
42x21 = 40x20 (2 teeth)
As you can see, your 3-tooth "rule" is just an avg.
I know on the hardest gears currently used it can be 5 teeth per cog.
Yes definitely be careful with this "rule". It's really only applicable for stuff in the 84-100" range, applied anywhere else and you're gonna have a bad time