http://sheldonbrown.com/gears/
Set to "MPH @ 100 RPM" (or "KPH @ 100 RPM"). Once you know your speed at 100 RPM, it' pretty easy to calculate your RPMs at other speeds.
E.g.: If 15 MPH = 100 RPM, then new RPM = [X/15 * 100], where "X" is the new speed.
If your new speed equals 16 (meaning, "X"), then your new RPM = [16/15 * 100] = 106.7 RPM.