(Another) Chainring question for the cycling experts....

Check your gear ratios here

http://www.sheldonbrown.com/gears/

In old-school theory, you should not be using the big-big combination or the small-small combination. I'm not sure how much it actually matters, but running your set-up through the calcualtor above will tell you what gear combinations give you roughly equivalent gearing.

Actually, there is a difference. And the extreme propellor-heads can even say why that is and how much of an effect it produces.

But to my mind, the differences are WAY down on the scale, such that if I did one more high-intensity ride it would make more of a difference on race day. So I don't worry about it.

But do avoid cross-chaining.

So assuming 53-39 front and 25-12 rear with 90rpm and 170mm crank

39-12  =  23.5mph
53-25  =  15.3mph

So basically ridingin both big front and rear and keeping the same cadence you lose 8.2mph.  If you rode 53-16 you would maintain 23.9mph so pretty close. 

Hope that make since.  I have heard some people say they stay in the small ring in front until they can stay around 20mph.  Personally I like to say where I have the most gears available in the back just incase I need to climb.  Sorry but I doubt I can explain this without actually showing you.  But I do prefer the front in the big gear.