Originally Posted by
chado445510
yes, i believe the rule of thumb is one tooth in the rear = 3 teeth in the front
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.