The tach needle itself is really a meter movement. Attached is a graph of the approximate resistance vs. the approximate needle position with +12v input. The current draw is from 2mA to 10mA around the top of the scale. Obviously if you lower the resistance it will draw more: at only 480 ohms the needle instantly pegs and it starts drawing 19mA which can't be good for the coil.
There is a lot of "bounce" when voltage is applied which makes sense given that the coil itself is an inductor. The output of the monostable multivibrator circuit will be a series of fixed-duration pulses, which will tend to "smooth" the meter movement-- the integration of the pulses is in the meter itself.
