math of ratios sorted out if you go here:
http://sheldonbrown.com/gears/
Point of sale when you buy a new bike,
the swapping of cassettes can be done then while the parts are all new
for the lowest cost..
Maybe a junior racer block will be better
they start with a lower high gear to keep junior's knees
from going out too soon in their life.
by trying to push the gears pros use for the 40+ mph finish sprints.