Agreed- simple laser tachos can be found most places- stick on a nice refective spot (as ed said) but you need the rest of it to be non reflective, cos nice shiny stainles discs will confuse the thing into reading 10000000rpm. As for wind loss, and other frictional loses, it should be possible to put an exact figure on that, then you can calculate how much energy is being disipated, and so how much work your motor is doing. Obviosly more friction an wind resistence results in more current being used by the motor, which is not good for anyone. Aerofoil spinner bars anyone?