This article is a quick reference guide to gear ratios on a bike:
(a) the distance that you will travel for one pedal turn on your bike
(b) the speed that you will be travelling for any given gear combination on your bike (shown separately for kmh and mph)
All figures are calculated using a tyre circumference of 2110 mm, and speeds are calculated using a cadence (pedalling speed) of 80 turns per minute.
These are useful for a variety of purposes, including deciding whether it is worth changing your gears to have bigger or smaller ring either at front or back.
Simply find the number of teeth on the big ring in the left column and the number of teeth on the small ring in the top row.
Metres travelled per single turn of the pedals
Speed in kmh when pedalling at a cadence of 80
Speed in mph when pedalling at a cadence of 80