OK, I finally found a bike to convert (an Astra "Tour de France" - $30). It has a 51/40 chainring set up, and I'm not changing them. My problem is that I bought a 16T cog and I think I want 15. But, the Bike Church only has 16 and 14T Dura Ace cogs. Should I shell out the extra $7 to get a 15T Surly cog? I really don't know, seeing as a 40x16 is 67.5 gear inches and 40x15 is 72. The gear ratio of a 42x16 is 2.6, while 40x16 is 2.5 and 40x15 is 2.7. I like the idea of a stronger gear, but money is kind of an issue, and I'm gonna head down town to do the swap pretty soon. Please help.