The numbers
(53/39, etc
) refer to the number of gear teeth on each end. In the case of the crank, it means that your large chainring
(the "gear" attached to the crank
) has 53 teeth, and the smaller one has 39. In the back, it means that your smallest cog
(the "gear" on the back wheel
) has 12 teeth, and the largest has 25
(and there are standard steps between that will be fairly common
).
Larger numbers in front means more wheel travel per pedal revolution
(or "harder"
). Smaller numbers in the back mean the same thing.
The spacing between the cogs on the back wheel has become standard between the various companies, however they can't just be switched, as the splines on the hub are different between the difference companies. So you could put a Campagnolo wheel onto a bike that otherwise runs Shimano shifters, and it'll work okay, but you can't put a Campagnolo cassette
(the set of cogs on the back wheel
) onto a Shimano hub, and vice versa.
If you're a strong rider, or train and race in fairly flat terrain, then the switch from a 12-25 to an 11-23 might be beneficial. I've ridden everything from a 54/42 with an 11-21
(when I was bike racing a LOT
), to a 50/38 with 12-27 in back. Just depends on yourself and your riding.