QUOTE=ryan_rides;21289015]Okay it might be great for someone who doesn't want to ever go near 30mph but it doesn't work for me. I had to replace my rear cog(15t) a few days ago to a 16t(already had it). It's a dura ace cog and lock ring. Should I buy another rear cog and lock ring? Or should I buy a new 52t chain ring?[/QUOTE]
A lockring is a lockring. Cog doesn't matter. It just needs to match your hub threads. No need to get another. (Exception - if you are going to very small cogs, ie 12 teeth; then you need a bell shaped lockring so the chain doesn't ride up on it.)
Eur-Asian cogs are just as good as Dura Ace and might be cheaper or easier to get.
Ben