Common question: How do you calculate "gear inches"?
Correct answer: Who cares? They are irrelevant.
The correct way to calculate the mechanical advantage between two gears on a chain is by division. Riding a 53-39 up front with an 11-25 out back? Your max gear is 53/11 = 4.818 and you minimum gear is 39/25 = 1.56. Tooth pitch is the same on both gears, so that and all the 2's and pi's drop out of the equation. There is absolutely no need for some semi-arbitrary factor of 26.39 to be included anywhere!
</irrelevant, arbitrary rant>
Correct answer: Who cares? They are irrelevant.
The correct way to calculate the mechanical advantage between two gears on a chain is by division. Riding a 53-39 up front with an 11-25 out back? Your max gear is 53/11 = 4.818 and you minimum gear is 39/25 = 1.56. Tooth pitch is the same on both gears, so that and all the 2's and pi's drop out of the equation. There is absolutely no need for some semi-arbitrary factor of 26.39 to be included anywhere!
</irrelevant, arbitrary rant>
