Rear cogs You have a choice of using either the standard fixed cog, or a freewheel. The fixed cogs are precision CNC machined, and are available in 13, 14, 15, 16, 17, 18 or 19T size.
The freewheel is only available in 16, 17 or 18T size.
Check out the Speed Calculator. What chain ring and cog combo is best for you?
Gearing Chart for 700C Wheel
Gearing Chart for 650C Wheel
Chainring and rear cog tooth sizes: This is all about gear ratios. As with a lot of things, the size of the sprockets is a personal choice. The standard 46/18 on the 700C wheel (44/16T for the 650C size) provides a generally popular mid-range gear ratio, allowing a flat road speed of 18.5 MPH at 90 RPM, small enough to climb all but the toughest hills, and not too small for the downhill ride, to keep RPMs from getting too high. See gear charts to the right for possible options.