The first thing to do is to turn the output of the hall effect sensor into a decent pulse wave form of stabilised voltage. Depending on the wave form at this point, a simple capacitor/resistor integration circuit would work. Basically every time your pulse comes along , suppose the capacitor charges up by 10mV per pulse , then discharges through the resistor by 8mv in the gaps between the pulses. at low revs you would be getting 800 X 2 mV times a second, at 4000 revs, 4000 X 2 mV times a second. Then you need a circuit to switch at a certain voltage level, to activate the overdrive solenoid. There is a lot of fiddling about to find the pulse width (2 degrees?, 10 degrees?) and the voltage level from the sensor. Then you have to set the CR time constant correctly, then work out a way of putting in some way of getting the overdrive "in" speed and the overdrive "out" speed(when you slow down), correct.
Frank