I think now that the heating of the motor will be slightly different on this waveform because of the motor's I-squared-R loss but if most of the input power goes into producing mech. energy then this effect will be minor.
If you know the thermal resistance of the motor (can be guestimated based on shape and volume), surface temp. (use a thermistor) and power in, efficiency could be determined to some level of confidence.
If no thermistor, after a good workout for the motor use your "digital probe" and check the chart below
no burn at 42C
burn in 30 sec at 54C
5 sec at 60C
1 sec at 71C
If
you monitor motor power draw and
drive the car up a ramp and
it takes 1 second to go up 1 foot and
the car weighs 1 pound
then
the car is putting out 1/550 of a hp
{else
it's not!}
As motor effic. drops Hero999 becomes "more right" and the OP would need to use a linear regulator.
I know these little wall 'formers are =>65% efficient and my fist-sized pond pump is 1% efficient at raising water.