jock, I was slightly incorrect, I have a 14 and 15 cog for my track bike. 48x14 is pretty big, 92 gear inches, equivalent to about your 53x15.5. Changing cogs is easier than rings so start with the 16, then go to the 15 or 14 if you need to. Of course you'll need the tools to change cogs with if you don't already.