Lower Gearing increased performance

Looking at the equations it seems that DC motors are most efficient at about 80% of their top RPM?
I think they put out most most power at half their top RPM (torque x rpm is a max at this point) but the motors use less battery energy at higher rpm due to back emf so its actually more % efficient nearer the higher rpms

the 3-axis plot (efficiency vs power vs rpm) of each motor is different (even for motors of the same type)
it varies with (and what casues the loss to increase):
whether or not the DC motor has interpoles if it has powered fields (if no interpoles, losses increase with torque/amps)
what the bursh advance is (ideal brush advance changes with both amps and rpm. miss-advanced, losses increase with torque/amps)
the quality of the brush and commutator (increases with torque/amps for resistance loss, increases with rpm for friction loss))
the resistance of the windings (increases with torque/amps)
the quality of the magnetics (increases with torque/amps and rpm)

thus, unless you are talking about a specific motor (and test it), you wont know what the 3-axis efficiency plot will look like (or even where the most efficient operating point is).
Once you have the data, you can work out approx what conditions the motor will be most efficient by which losses are most dominant (losses that increase with torque/amps or losses that increase with rpm).
A well designed motor for a specific application will show greatest efficiency at rated power at rated rpm.

to work out how many volts of back-emf you get on a 24v 250w motor, use this equation (its still an approximation to keep the maths simple):
back emf = 24v - motor current * winding resistance

Matt