My fixed gear conversion is 40/15, which cruises nicely at about 18mph on the flat. Don't ask my cadence - I'm not that anal about riding. It just feels comfortable.
I established the gear ratio by riding my road bike on group rides at one gear, and finding the most comfortable one, then using Sheldon Brown's site to convert it for my Schwinn's existing 40 tooth small chainring.
Wouldn't it be easier & cheaper to change the cog instead of the chainring?
Hope that helps.
BTW, that Roger looks like a serious gravel trail weapon. Now you've given me a bug, dammit.
