a higher number of teeth on your spur equals a lower overall gearing wich gives you more bottom end....
a higer number on your bell gives you more top end.. .
End of story..
the larger gears turns less times to the bells single rotation.. equalling more torque...
example.... 1 turn on the 13 bell.. would equall.. 1/4 turn on a 48 tooth spur..
1 turn on a 13 bell would equal.. 1/16 of a turn on a 62 spur..
hope this helps...