http://www.sheldonbrown.com/gears/ Have a look at that^ for example...a typical stock gearing would be 26" wheel, 170mm cranks, 18t front cog, 16t rear that gives you 29.7 GI ( gearinches) Normal mod gearing is usually 18:12 or 18:13 although one of my friends who's a begginer runs 20:18 which is a joke to be honest