Power is not the only specification of a motor. It has more than just volts and watts. For example torque is an aspect you need to consider and the higher the rpm, the lower the torque, given no change in power.

Here you have all formulas for motor : http://www.elec-toolbox.com/Formulas/Motor/mtrform.htm

And here is something similar to use an rc motor on a bike and it seems more complicated than you probably think : http://endless-sphere.com/forums/vie...hp?f=28&t=8503