Earlier on in this thread, it was suggested that a 4046 PLL with a divider in the feedback should be used - it's a classic frequency multiplier.
This was 'rejected' as it won't work down to 0MPH, well neither do mechanical speedo's.
The 4046 solution could be made to work quite easily, as I remember, the 4046 has an 'out of lock' output - simply use this to mute the outgoing oscillator signal. As the car stops the input pulses stop, the 4046 loses lock and the output is muted - the 4046, as with all PLL's, will only lock over a certain range, but it doesn't need to lock at very slow speeds (0-5MPH), mechanical speedo's don't read that low.
By using a comparator on the VCO control input, you could actually preset a point where the output gets muted.