Ultrasonic sensor

This is probably the best quality of the sensors i think, and im setting myself up for and idiot moment here but anyway tonight im trying to build a little "motor efficiency" meter by using the u.sonic proximity sensor to measure the RPM of the motor when shaft is connected to a propellor of known mass dimensions etc etc and taking the difference between the mechanical kinetic energy and the potential difference between motor terminals

sillyness?

arduidiot: sillyness?

Perhaps, perhaps not: I haven't the faintest idea what any of that means.

like the difference between the electrical energy supplied to motor and the amount of work the motors putting out