Food for thought...
For many years now, electronic speedometers and tachometers have been used in many types of highway vehicles. My experience has been with heavy-duty trucks, so I'll explain the signal generators (pickup mechanisms) used therein.
The most-commonly used pickup is basically a coil of wire wrapped around a permanent magnet in "rod" form. This in then ecased in a housing that allows for convenient mounting within proper proximity to its triggering device. In most cases, the tachometers used the engine flywheel (starter) teeth for triggering, and the speedometers used a toothed reluctor wheel that was mounted to the transmission output shaft in place of the traditional speedometer drive gear.
The operation of the sensor is simple -- the approaching and departing teeth on the trigger wheel would cause a signal with a sine wave form to be generated in the sensor's pickup coil. The frequency of the generated signal is dependent upon the rotational speed of the trigger wheel.
There are many aftermarket signal generators available. Take a look **broken link removed** for the offereings of just one company... many different drive methods are available, including types intended to be driven by the factory drive cable already installed.