Horse Power Loss ?

Rotational Mass is a big one....Wheels, tires, axles, drive shaft, tranny, tq converter/flywheel when spinning combined have a huge amount of inertial kinetic energy all of which resists a change in speed either accelerating or decelerating. To increase the rotational speed of all of these items takes a significant amount of energy which never gets to the ground. This energy is seen as a loss when comparing Flywheel HP to Wheel HP.