Due to cross-chaining issues, I came up w/ a formula for the actual # of usable gears on a bike, which is less than the stated number by the manufacturer.

For instance, on my bike, which supposedly has 24 gears (3 in front, 8 in back), I only actually have 16 usable gears, and not 24.

The formula can be expressed in this form:

(# of speeds by manf) - (# of rear sprockets) = actual # of usable gears
ex. 24 - 8 = 16 usable

Note that this formula only works if you have 3 sprockets in the front, and any number in the back.