I have a 38T and a 42T chainring for single speed racing in Utah. With the 38T my range of rear cogs goes from 16T-19T based on chain tension, the length of my drop out slot and the tolerance of my rear tire and frame. 80% of the time I race on 38T/16T, leaving my quick release at the end of my drop out. I use the 19T on two different hilly courses here in Utah and whenever its snowing. There was one race I was tempted to run 42T/17T because it was so flat and my chain would be too slack if I ran 38T/15T. This season I also switched out my rear 8.4 V-Brake for an Avid Shorty Ultimate because it's much quicker and easier to adjust each time I switch out my cogs. I still rock the V-Brake up front.